It also demonstrates what superior people Rozy's aunt and grandparents are. Ralph loves to shit on any family members of people he hates, their spouses, their kids, whatever he can lash out at like the scum he is. He's done nothing but curse his daughter's maternal family for the last six months like the lowest, vilest, trash imaginable.
If the Morris family was anything like Ralph they wouldn't be much interested in interacting with a child sired by Gunt, or help out Amanda, but instead they open their arms to give Rozy the love, security and family her scumbag father reveled in depriving her of.
Ralph tried his absolute hardest to alienate the Morris family from May and Rozy, but utterly failed in the end. Ralph will never have a family, he's not capable of it, he will just have some genetic connections that despise him, if they even know he exists. The only person who ever loved Ralph was Sandra, the woman he treated like garbage, and she's dead. With Sandra the last vestiage of family Ralph would ever have died with her.