I'm not going to let my sister in my baby's life
What purpose would that serve?
I'm going to cut her out of my baby's life.
If there is no reason to do so, it may not be a great idea to cut her out. Your baby probably is going to most likely want to meet their aunt at some point.
I have a secret plan that I'm going to kill her.
Killing your sister is going to leave your baby without a mother, as you would be in jail. In is understandable to dislike your sister if there is bad blood, but try to make the best decision with your baby's future in mind.