K.R.
With a psych background, I have learned that it is important to tell children what they can do instead of what they cannot do. For example: please share with your cousin, instead of don't take her things. Also, it is important to demonstrate ways to share. Get down on her level and show her how to share and continuously reward her for the good things. It sounds as if she is jealous and stating things in a "no" sort of way will only push her in that direction. She continues to learn how to interact and may be intimidated by what she cannot control...the fact that the baby gets more attention (positive attention). So, in order to get attention, she may be acting out to get any attention at all, whether it be positive or negative. It is important to say positive, rewarding things to her when she does so and not to label her as selfish. If a child is continually told she is selfish, how does one expect her to change? I am not saying this is done, just food for thought.
The main issue sounds like jealousy, so focus on anything positive, and she will like that attention so much that she won't be able to help herself from doing positive things for positive attention. I know there are many aspects to this issue which both you and your mom face; this addresses just a few and hopefully helps. Also, since this is such an issue, could you set up a reward system for sharing? Just a thought. We have to teach them their actions and can't expect them to come so naturally. the fact that you're concerned shows that you are a great mom. keep up the good work. and try at all costs to be positive....your mom too.