The priest has been taught that the RCC is the only way and will probably try to guilt-trip you into going there. Your mother-in-law could still be sending demons at you by magic, or they might have just come from her without her intending it, since she left the door open for them. The best answer I can think of is simply love, prayer, and faith. She's hurting herself more than you by using magic like that, so pray for God to shield her and make her witchcraft lose it's power.
I believe if you keep reading the bible and praying, the demons will lose their power and eventually quiet down. Be sure to get some good sincere christian friends to pray with you and for you. (Here on the forum I'm sure many are already praying, I certainly am.) The pastor might or might not be a help, there's good and bad pastors out there, and some are more skeptical of the supernatural than others. I don't think church attendance is high (if anywhere) on God's priority list, he just wants us to fellowship enough to be strong in His love. The Church is really all believers, not just one set of them.