I would cut your losses if I were you. Being in a relationship with someone who comes with a difficult family can be soul-destroying.
I was close to my ex for years and regarded him as a good friend. He's recently remarried and his new wife worked hard to drive a wedge between us and has succeeded. It's a shame but he's made his bed and now he can lie in it.
I would never discourage anyone from staying friends with a former partner. Friendship is much better than animosity.