I also questioned the baby’s paternity. But I think it wouldn’t have mattered. Paulie wasn’t their son/brother but completely a loved family member. Even if it was revealed the baby wasn’t his, he could have raised it because Donatella wasn’t capable of doing so; Paulie always rescued her and would step up for her and the family. Since his family rejected him when he came out, he wasn’t going to let this baby be parentless and grow up with out unconditional love.