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What is a Half Sibling?

Do you want to know what a half-sibling is? In this post, you will learn the definition of half-siblings, whether they are considered immediate family, and if they share DNA.

You will also find out:

  • How closely related half siblings are
  • The difference between a half-sibling and step-sibling
  • How much DNA half-siblings share

“Half” relationships can be very confusing, but they do not have to be. Once you learn the basics, it will all become clear to you.

What is a Half Sibling?

It is important to understand how half-siblings are related, especially if you have half-siblings yourself, or if you enjoy family tree research. The descendants (i.e. children, grandchildren, etc) of half-siblings have a unique “half” relationship, too.

What is the definition of half-siblings?

Half-siblings are two people who share a single parent in common. Half-siblings may share either a mother or father, but not both.

A male half-sibling might also be referred to as a half-brother, and a half-sibling who is female can be a called a half-sister. However, half-sisters and half-brothers can also be called “brother” and “sister”, as adding the “half” is unnecessary.

Occasionally, the term “maternal half-siblings” is used to refer to two half-siblings with a shared mother. If siblings share only a father, the term “paternal half-siblings” can be used.

The most recent common ancestor of half-siblings is the shared parent. Because of sharing one parent, half-siblings share 50% of their ancestors in common.

Are half-siblings considered siblings?

Yes, half-siblings are real siblings. Even though half-siblings share one parent instead of the two parents shared by full-siblings, they are genuine sisters and brothers to each other.

In fact, half-siblings might even feel closer to each other than some full-siblings, especially if they grew up in the same household together.

Are half-siblings considered immediate family?

Yes, half-brothers and half-sisters are considered to be immediate family members. This is because the genealogical relationship is that of siblings, even though they share one parent instead of two.

There are many other close relatives that are typically considered to be immediate relatives. In addition to half-brothers and half-sisters, full-siblings, siblings who were adopted into the family, and step-siblings are immediate family.

Are half-siblings more related than cousins?

Half-siblings are more closely related in a genealogical sense than cousins, since half-siblings are very close relatives. However, half-siblings share the same number of ancestors with each other as they do with their first cousins.

Two half-siblings share 50% of their ancestors because they share one parent. Since the common ancestor shared between first cousins are two grandparents, first cousins also share 50% of their ancestors.

If the parents of the first cousins are half-siblings, however, the cousins are technically half-cousins and will share only 25% of their ancestors.

What is a half-sibling versus step-sibling?

Half-siblings are biologically related, while step-siblings are related through marriage. There is usually no known biological relationship between step-siblings, unless the parents are distantly related to one another.

Even though half-siblings are not usually biologically related to their step-siblings, they can still have a very close relationship. Many people do not make a distinction between their biologically-related siblings and their step-siblings in regular conversation.

How do half-siblings show up on a DNA test?

Half-siblings will show up as close relatives on autosomal DNA tests, such as AncestryDNA or 23andMe. On Ancestry DNA, the half-sibling will be in the “Close Family” category, while on 23andMe, the half-sibling may be identified as such.

Two people who are half-siblings, whether male or female, will share between about 18-33% of their DNA. In centimorgans (cMs), this measures out to an average of about 1750 cMs, with a range of about 1300-2300 cMs.

Half-siblings who share the same mother will also have the same mitochondrial DNA (mtDNA). Half-brothers who share the same father will have the same Y-DNA, so their haplogroup should be identical.


I hope that this post has helped you understand the definition of half-siblings, whether they are considered immediate family, how closely they are related, and whether they share DNA.

If you have any questions about something that you read in this post, please join the discussion below.

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