Considering adoption

Im married and were trying for a baby however we ran into issues its been a year next month. My sister has a baby her and the father arent together and she lives with my mom who raises the baby. My sister is pregnant again. My sister is addicted to drugs doesnt work. Shes about 6months pregnant and "just" found out. She doesnt want to keep it and an abortion is no longer an option. Her ex wants her back and the new baby but isnt allowed at my moms house. Ive been considering asking her if she would let me adopt the baby. I would ask for a closed adoption of course she would see her"niece/nephew" but her ex would not be allowed in the childs life period. (Her ex beats her threatened to kill the baby they do have and is just a pos) im worried if i did adopt hed try to find me or hurt me or show up at my home which i know sounds selfish but i really cant have issues like that where i live or id be evicted. I know this group is mainly for women who are adopting out their babies but would love some advice from people who have been on the other side.