My dog is acting like she's been abused!?

Yes your dog can sense your cousin's aggression but if she is growling at everyone but you that is not the problem. I'd say she has been abused in her past, and because you treat her well she has formed an unhealthy attachment to you and is jealous / protective of you against anyone else. She is probably not naturally a dominant "alpha" dog but feels she has to try to be because you're not giving her the vibe that you're in control of her. This is probably also why you can't trust her at home for 6 hours, which at 3 yrs old you should be able to do - she gets separation anxiety and acts up when you're away for too long. There are techniques for dealing with all this, I would strongly suggest getting professional help in this case because it sounds like she might bite someone, which is dangerous, mostly for her because it might end up with her having to be put down.