I'm a Catholic, and I attended a CCF worship twice, after which I told my priest friend about it. And he said, well, so when you go to mass, and say the Nicene Creed (I believe in one holy, Catholic, Apostolic Church, etc etc), what does that mean to you? Does it still mean anything? If you profess that you truly believe in one, holy, catholic, apostolic church, why go to another Church? If that doesn't mean a thing to you, then perhaps you need to reassess your faith.
It kind of knocked some sense into me. Oo nga, how can I claim to be Catholic, stand in Church before God and say all those nice things, and THEN go to another church the following week? It doesn't say "I believe in a church in all its wondrous, various forms." Hehe. It says "I believe in ONE holy, catholic, church." So if you want to be super formal about it, the Church does say that there is only ONE true Church, the Church that Christ himself instituted on earth. Maybe that's what your sister meant when she said "bawal" for Catholics.
Having said that, I've come to believe that we all have different processes in finding God. Maybe your path to God is through CCF, I don't know. But yes, be true to yourself nalang, so you can stop being conflicted.