my sister is pregnant

i absolutely hate her boyfriend and i neevr ever will accept him i told her many times i dont aprove of him in very strict way told her i wouldnt be a part of her life if she went on with this.
he cheated on her very early in their relationship he is best frienda with her best friend who is a horrible poisonous snake who will do anything to get what she wants a spoiled from birth with no regards for values or sensitive to others feelings like a sociopath. she lured this relationship. the guy and the best friend together will destroy my sister and my family. its all planned. i decided to cut relationsships with my sister for the greater good because their goal is to subvert my family through the baby bring the father to the spotlight and the father bring his evilness..i am.worried though the damage it can do to my family. they live a bit far but often come to the city. i said i dont want him in our home but idk how long it will last until he comes for christmas easter.. someones birthdah... sundays etc im.really worried it might escalate. im worried i might end up beating him up too or getting in trouble.