S.K.
My brother was in a similar situation. My oldest niece is not biologically his, but he raised her from the time she was 1. Her biological father was not in her life AT ALL physically, but did pay the ordered child support. My brother and sister-in-law decided to do a legal name change, so Ciara does have the same last name as her mom and dad, and when her brother and sister came along a few years later... the same last name as them. It worked out really well for them!