I empathize with your situation. I know some sociopaths. They don't change. I suggest reading books, and articles about sociopaths, to help you recognize the traits you see in your cousin. If your going to get back together with her romantically, you need to drop some of the typical romantic expectations that we place on our lovers. You need to accept her behaviours, and be willing to tolerate her. You need to accept her as she is. So you need to gauge how bad are her behaviors? What is the worse thing that can happen?
Fortunately for me, the sociopaths that I know, I'm not romantically involved with, which makes it easier to accept them as they are, and try to handle them, or work around them. I have no personal needs that I want them to fulfill, which is usually the case in a romantic relationship.
Things I've observed about myself and life as a result of all my relationships is that when we get married, or common-law, etc., we subconsciously place expectations on our spouse. We expect them to be smart, honest, trustworthy, and a helper to name some of the big ones. If you are financially dependent on that spouse, that can put you in a bad situation, if the person is not honest and trustworthy and dependable.
So you do not want to depend on her for anything. You need to put together a Plan B for you if things don't work out.
I've read that sociopaths and psychopaths can mellow out some as they get older. The main sociopath I deal with has mellowed out now that he's in his 70s somewhat. But he still does sociopathic behaviours on me, and I still have to deal with them, tolerate them, try to work around him, etc. It would be much harder if we were romantically involved.
If you're trying to handle a sociopath, you can't usually tell them you're doing that, because they'll get defensive and argue, even when they're very wrong. You want to do things that will avoid them abusing you. And you need to stand up to them if they are abusing you.
For example, sometimes my sociopath will start minimizing something he's done to me, and I tell him that he doesn't know how (whatever it was) effected me, and he has no right minimizing the pain he caused me. If you stand up to them, sometimes they'll back down on an abusive behaviour. There are things you can say and do to lessen their abusive words and actions towards you.
Learn what abuse is exactly. I read books, but I never really learned a good definition for abuse until I went to a women's shelter years ago and took one of their outreach programs. Because a women's shelter is on the front lines of abuse, they don't mess around with flowery words. They get right to the point. What I learned from them about how abuse works, was much better than any books I had read on the subject.
And you need to learn about how abuse works, because sociopaths abuse the people around them. If you know how abuse works, it will help you NOT make their abuse personal, so that it hurts you less in the long-run. And you need to have your own head on straight. If for example, you're suffering from abuse you've received earlier in your life, you will be more vulnerable to someone who abuses you.
I can see by what you wrote, that you're still in a lot of emotional turmoil over your cousin-love. If you associate with her in any fashion, take things slowly, perhaps don't live with her right now. Be her friend, if you want to, but have some time away from her in order to keep your strength up.
These are tips. There are no easy solutions to intense cousin-love or dealing with a sociopath. Keep in therapy if it's helping you.