need to save my nieces

Hello, I am new to this website. Also I am on a tablet so please forgive any grammar/punctuation mistakes. I am a 24 year childless female in akron, ohio. I am the oldest of 5 children. My 20 year old sister just had her 3rd child (3 different fathers). She has admitted to being a heroine user (prior to this last pregnancy, but we suspect she may be using again) and we are aware of consistent prescription drug abuse. She never went back to work after maternity leave in October and her current boyfriend/most recent baby dad is a 25 year old convicted felon with a known drug problem, no job, still married to someone with 4 other children. We (my mom and I) have told her multiple times that she needs to get rid of him and we would help her as much as possible. She just absolutely will not listen to us and refuses to do anything proactive. My oldest niece is 3, followed by a 14 month old and a 2 month old. These poor babies have been weighing on my heart. I dont want them to grow up in such dysfunction, they are angels. I have talked to my mom and I feel ready as a responsible adult to take one of the girls into my care. I guess I am looking for any concerns, advice, instructions, or any information I would need. I have not brought this up to my sister yet as I am afraid of her response. My mother has also decided she could take the other 2 babies. I am trying to gather as much info as possible before I spring this on her and/or call CPS. Please share any info that may be helpful! Thank you!