I don't know which 'mother' should be the one to look after them, but I do think that in nearly every circumstance it is better for a child to have a mother than not.
When you say “mother” do you mean female parent? It sounds like you’re happier if the babies are going to a female parent than two Male parents. In the scenario we’re discussing, were calling Mel the mother (she birthed them so is birth mother)- her sister wouldn’t be their mother.