It's normal for you to feel jealous right now. As long as you recognize what you are jealous about. It's not the baby herself, it's your mother lavishing her love on baby and you need her to show you love as well. Right? You can have a good relationship with your niece in spite of that.

Maybe your mother does not know how to deal with your illness, feels helpless, and thinks if she ignores it it does not exist?