Do Catholics believe in the Old Testament?

I have a sister-in-law who was a catholic but is now a Baptist who is completely anti-catholic. She claims that Catholics do not believe in the Old Testament and it’s prophecies and the Holy Spirit. She calls the catholic religion a ‘cult’. Please suggest a book or article that I can verify and be able to give her a clear cut answer. She refuses to read any catholic books. She so much thinks that we will all go to hell. I feel sorry for her and want show her how wrong she is about the catholic faith.

PS: not sure what the tags are for below!

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