That's not an easy situation to be in.
If C is already having trouble being nice, it shouldn't be long before J starts to see how C actually treats people.
First, if you can, you should try to avoid hanging out with C unless J is there. Second, try to find as much time as you can with J where C is not involved.
C seems to be a bad influence, it might be a good idea just to try and remind J that disrespecting adults, specifically her father, is not a good idea. Although, if C tries arguing with you, just stay calm and ignore her as best you can.
I think I'd try talking to J if I were you, just bring up topics that involve C and try and figure out what J thinks about C and about what C does. That sort of thing. This could be a good time to just mention the way C treats you at times. Whatever you do, avoid arguing with J. It'll just make both of you upset and possibly make the situation worse.
And I don't think ending your friendship with J will help. If your really good friends, losing her won't be worth the benefits of not having C around.
Hopefully this helps, and I imagine the others will have some good suggestions for ya
