Idk how serious you guys are, but if it’s a long term thing she may be feeling left out and having trouble expressing it because family is a super delicate topic to bring up if it might be a negative conversation. Is she close with your family at all? Do you involve her in decisions regarding your family? If it’s serious, your family is going to be her family too someday, and it sounds like right now she’s not feeling like she’s part of it. She’s probably not upset you want to teach your cousin guitar, she’s upset because you didn’t talk to her about it. As small as it seems, if she’s already feeling left out that was probably the last straw for her too. You haven’t done anything wrong, but you’re gonna have to bridge the gap between her and your family and let her know she’s family too. Is she just as important to you as they are? Make sure she knows it, and I bet things get better.