
Quoting
aardvarc
If two kids have two different fathers, why SHOULD the last names match? Unless one father legally adopts the other child, his name has no business attached to the child that isn't his. Last names get established based on the person whose DNA was chosen to father the child - not on who they live with, or who would like to be perceived as the father, how well or poorly a parent functions as a parent. Dad doesn't even have to prove that it's in the child's best interest, only that he IS the father. To challenge the presumption that the child should have his last name, the burden would be on you to prove that it's in the CHILD'S best interest to NOT have dad's last name (and unless dad's last name is Dahmer, Hitler, or Bin Laden, that's incredibly difficult to prove and thus in 99% of cases fathers prevail). Simply wanting a child to have the same last name as a half-sibling isn't enough to over-ride dad's rights.