the fact that he was called something different does not change that in islam there is no restriction on someone raising a child not biologically their own.
guidelines get broken everyday, if the concern is hijab then mamay chahay cousins are not mehram either nor are classfellows, but that does not seem to be an issue in society at large.
rest is easily manageable anyeways.
in the end I dont think god will have a huge issue with someone not observing hijab to the max if on the other side it meant that kids who would otherwise be exploited and abused got a good home life.