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?