Can half-siblings show different relationships to extended family?

Question

My husband has found a half sibling, first on Ancestry showing a match of 1542 cm over 77 segments, and then confirmed on 23andme with a suggested relationship of "half sibling". Ancestry only showed "close family - 1st cousin" so the 23andme test was done to confirm.

Neither of them knew their birth father, and both of their mothers have similar stories about where they met him, his background, etc. Both mothers are American with European background, but the birth father had arrived from a different country shortly before they were born (and DNA shows they both have almost 50% of this nationality, which their mothers do not have).

They are 4 years apart in age, so we had ruled out grandparent relationships. Based on the accounts of the birth father's age, aunt/uncle is not possible. But on 23andme, my husband and his half sister show different family relationships with extended family on that side.

For instance my husband has two "2nd cousins" sharing about 3%, while his half sister shows these relatives as 3rd to 4th cousins, sharing 1% or less. Another relative shows 3rd cousin to my husband but 5th to her. Is this likely to happen with half siblings, due to how the DNA is distributed? Or does it mean something else?

Answer 1

It can happen with any sibling relationship, due to the amount of DNA they inherited from the parent's parents.

I'm going to continue from here under the assumption that we are not including the sex-determining chromosomes (X and Y), only the other 22 chromosome pairs.

A person inherits 50% of their DNA from each parent. On average, a person inherits 25% of their DNA from each of their four grandparents. However, usually a person inherits 20 to 30% from each grandparent (still totaling to 100%).

Your husband and his paternal half-sister most likely inherited slightly different parts of their father's DNA. Which also means they inherited different parents of their paternal grandparents' DNA. Depending on the DNA the potential 2nd cousin inherited, that cousin could show up as some closer, or rather more distantly related, to one half-sibling than the other.

As an example, my mother and her known 2nd cousin (they share great-grandparents) share a less-than-average amount of DNA for 2nd cousins. Most likely because of this, Ancestry shows them as being 3rd cousins. (Unfortunately, I do not have either of my mother's two brothers on Ancestry to see how they compare to this second cousin.)

Because your husband and the sister are half siblings, the generational distortion for cousins could easily be greater than it might be otherwise.