I think your instincts are correct with this person. She sounds like a massive manipulator and she's proven to be a liar. Those two characteristics go hand in hand. Manipulators are always liars. They are also very nosy. Hence using Facebook to spy on you but not interact with you. They are masters at using people to get what they want and play many power games to accomplish that. Probably why she'll ask you about a Facebook post or some other topic out of the blue when she's giving the appearance that she does not care about it. You may want to warn your cousin to be careful of her. And please don't take offense, but I believe it's entirely possible she may be after his money. I've dealt with people like this and I unfortunately have one in my family. Only thing you can do is limit all contact with them. Give them as little information about you and your life as possible. Block them on social media so they can't spy on you. Taking information away from them is the only way to take their power away and get yours back. They are toxic, will never change and should not have a place in your life. Good luck.