Am i just overreacting.?

My sister recently had a baby some months ago. The little girl is absolutely divine and I enjoy taking care of her while my sister goes to work during the day. However, lately my sister has been behaving like a jealous spoilt brat. She also behaves as though she's the only person in the world to ever have a child. I understand this is her first child and her world revolves around her daughter, but sometimes when the baby looks at me, my sister goes "Don't look at her. Pay attention to me." I think its annoying. Not to mention my sister's new condescending tone to anyone who takes care for the baby especially towards me. Her child is not the first I've taken care of and I treat my neice as she is my own child and I take pride in caring for children. My sister doesn't take advice as for instance when I told her the baby is getting sick and she defensively responded "the baby's not getting sick!" then the baby got sick. The most annoying thing she has come to do now is the phrase "Ever since I became a mother," in kind terms it grinds my gears everytime she says it. She says it with this self righteousness and unneccessary need to feel important. Half the time I don't know how to interpret her reasons for behaving this way and at the moment I kind of don't want to deal with her anymore. Should I address the situation with her or leave her alone.