Think of St. Monica, who prayed for years and years and years for her wayward son, Augustine, who became a saint. In addition to your prayers, make sure that YOU model Christ in your life, so that when your family members look at you, they see a person who has the joy of Christ within them. Joy and inner peace are attractive to people.
Sometimes family members are the hardest people to evangelize to—lots of extra baggage, so to speak. Often, people are resistant to a message (whatever the message may be) if it comes from a family member. Remember that—that’s another reason why it’s so important for you to model a joyous Catholic life to them.
I’m in a similar situation in that I am the only one of six children who is Catholic—everyone else fell away (I did too, but returned). I am making some headway (or rather, the Holy Spirit is!!) with one sister who is going through some self-induced rough times: her situation creates more receptivity to what I have to say than there was before. I have sent her books and other material that I thought would be relevant to her situation, and she actually has read them and liked them. So, there’s hope there…anyway, don’t give up! Pray and fast for them.