Half Siblings Explained (All Questions Answered)

This article gives simple and clear explanations of the half sibling family relationship. I also answer some other common questions around half siblings.

Let’s start with the basics!

What Is A Half Sibling?

You share one parent with your half siblings. You either have the same father or the same mother.

Full siblings share both father and mother. When the relationship involves only one of two parents, the siblings are legally referred to as half brothers or half sisters.

Here is an illustration of one type of half sibling relationship. In this case, the father has sons by two different women. The sons are half brothers.

Of course, your half siblings can also be on your maternal side. In this scenario, your mother has a child by a different father to your own.

You and your half siblings are of the same generation.

You share one common parent and two common grandparents. In contrast, full siblings share four common grandparents.

Do People Always Say “Half Sibling”?

Socially, many families “don’t do halves”. In other words, they simply refer to half siblings as their brothers or sisters. They don’t call attention to the distinction in everyday life.

This is likely when all family members know each other from childhood.

In contrast, there are other families where children are raised with little or no contact with their half siblings.

They may use “half sibling” to make it clearer that it is a socially more distant relationship than with their full siblings.

This is an individual choice.

What should you call other peoples’ half siblings?

Are you unsure of how to refer to the half sibling of a friend or acquaintance?

It’s best to listen first to what they call each other. But you may be in a situation where you’re making introductions without knowing the preferred terms.

All I can do is tell you how I operate.

I would start without the distinction e.g. “this is Jim’s sister Mary”. Either Jim or Mary will quickly correct you if they prefer a different label.

Half Nieces And Half Nephews

If your half siblings have children, what are their kids to you?

Genealogically, they are your half nieces and half nephews.

It’s rare in English-speaking cultures to reference the “half” in this relationship. Most of us will simply say nieces and nephews.

What’s The Difference Between Half Siblings And First Cousins?

Half siblings are a closer relationship to you than your first cousins.

You share a parent with a half sibling. So, your closest shared ancestor is one generation away.

In contrast, your closest shared ancestors with first cousins are two grandparents. These are two generations away. If you want to explore more, check out our separate article on first cousins. It is full of diagrams and answers to common questions.

“Real” family

I must admit it surprises me when people wonder if half siblings are “real” family.

If you consider that your close cousins are “real” family, then it seems odd to think that half siblings wouldn’t be too.

However, I understand that social factors come into play. People may grow up knowing their first cousins far better than their half siblings.

Are Half Siblings Blood Related?

Your half sibling is a blood relative because you share a common parent.

If the sibling relationship is solely through marriage, then you are not blood relatives. Instead, you are step siblings.

Let’s look at this in more depth.

Half Siblings Versus Step Siblings

We’ve explained half siblings, so let’s take a look at step siblings.

Your step siblings are children from a parent’s marriage with whom you are not blood related.

How could this happen? It’s easier to explain with a specific example.

Let’s say that your parents divorced and your mother married a second husband. This new husband also had a prior marriage with children.

The children from that prior marriage are not blood related to your mother. She is their stepmother, and they are your step-siblings.

This situation is illustrated in the infographic below. We’ve also included your half brother from your father’s previous relationship.

You may be wondering if there is a genealogical relationship between the half brother and the step brother in this example? The answer is no.

This is speaking genealogically. However, socially there may be different choices.

It’s no longer unusual for marriages between two divorced couples with children from prior marriages.

The offspring in these families may choose to refer to each other as cousins or siblings.

Do Half Siblings Share DNA?

Half siblings share about 25% of their DNA.

The range of shared centimorgans is from about 1,160 to 2,435 centimorgans of shared DNA.

(Cm as reported by DNA Painter).

The reason for the variability in the centimorgan range is that people inherit different amounts of DNA from their common ancestors.

Using DNA websites

If you’ve used one of the big commercial DNA testing services, you may be researching a list of DNA relatives who also tested with the same company.

Be aware that the higher end of the centimorgan or percentage range for half siblings will overlap with the lower end of full siblings.

Similarly, the higher end of first cousins overlaps with the lower range of half siblings.

So, be wary of the labels assigned by the DNA website. That “first cousin” may actually be a half sibling.

This is explained more in our article on how half siblings appear with Ancestry DNA tests.

Can half siblings not share DNA?

Is it possible that you don’t share DNA with a half sibling? The answer is no.

Half siblings will always share a significant amount of DNA.

If you and your half sibling have both tested on sites like 23andMe or Ancestry, you should be able to see each other listed as DNA relatives. 

Are Half Siblings Considered Immediate Family?

In many jurisdictions and circumstances, half siblings are considered immediate family in legal or policy matters.

However, you should always check with the specific organization (or state authority).

For example, you may want to take special leave to attend a funeral. You should examine the list of people covered by company policy.

Sometimes they will reference step siblings but not half siblings.

If that’s the case, I suggest you have a quiet word with your manager or HR department. The person writing the document may have assumed that the phrase “sibling” covers half siblings.

I’d be surprised if the courtesy wasn’t extended to your family situation.

Do Half Siblings Look Alike?

Anecdotally, I can tell you that I know half siblings who resemble each other more closely than they look like their full siblings.

However, you’ll also encounter many half siblings who bear no family resemblance with each other.

There is no way of predicting whether half siblings will look alike or not. They both inherit a percentage of DNA from their parent – but they don’t inherit all the same regions of DNA.

Due to the random nature of inheritance, there is no way of predicting who gets what.

Can You Marry Your Half Sibling?

There is no jurisdiction in which marriage is allowed between half siblings who are blood related. This includes every state in the United States.

The only exception is when the relationship is through adoption. In other words, one of your parents legally adopted a child with someone else.

However, marriage in these circumstances is also illegal in many jurisdictions.

Frequently Asked Questions

Do half siblings have the same last name?

Half siblings may not have the same last name.

The most likely situation for the same surname is when the relationship is through two marriages of a common father.

Can half siblings be twins?

Identical twins could never be half siblings. This is because they were formed by the same egg and sperm. They had to have the same father.

There are rare examples of fraternal twin births where the children are half siblings through different fathers. This is known as superfecundation.

Can half siblings inherit?

This depends on the legal jurisdiction. Some jurisdictions specifically include half siblings in matters of inheritance.

Margaret created a family tree on a genealogy website in 2012. She purchased her first DNA kit in 2017. She created this website to share insights and how-to guides on DNA, genealogy, and family research.

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