Distinguishing Half Siblings from First Cousins using DNA?

Question

My husband is adopted and we were given non identifying information by the adoption agency. After doing a DNA test on ancestry we have found his mother, unfortunately she is not open to sharing any information. We found her through a half niece match, her numbers were 814cm 31DNA. Recently he has been matched with numerous cousins on his fathers side two of them are 1st cousin matches 807cm 37DNA and 561cm 29DNA. Here lies the confusion, this family had 10 sons, none whom stayed with the women they impregnated so the matches also know very little about their bio father's other than names and birth year. Well the first cousin whose numbers are 807cm 37DNA fathers information matches my husbands non identifying information exactly! Height, eye color, occupation, country born and birth year.

Could this possibly be his half sister?

I have looked at all the charts and it seems like she could not but with everything else matching I dont know how she could not be.

Answer 1

No, this is a 1st cousin, not a half-sibling.

According to the Shared cM Project, half-siblings average 1783 cM, with 99% in the range 1317 - 2312 cM. First cousins average 874 cM, with 99% in the range 553 - 1225 cM. Note that there's no overlap between these 99th percentile ranges, so a given sample is either one or the other.

The good news for you is that you can eliminate that particular 1st cousin's parent as a candidate, and maybe keep looking for matching samples from other cousins in that family.

As to the matching non-identifying information, down to the birth year, it's possible that the biological mother either intentionally misidentified or honestly didn't know who the biological father was. When reviewing records, it's important to consider under what circumstances the informant might have provided misinformation, either intentionally or inadvertently.