Want custody of sibling

My mom is a drug addict. We have been going through trials and tribulations for years. It has gotten so bad that last year she overdosed, did not seek the help she promised to get. And now again overdosed 3 days ago. I have 2 younger siblings ages 17 and 14 living with her and their dad. Dad does not do drugs but allows her to do stuff in the house because its better than being on the street God knows where and he supports her habits. They have built a terrible cycle together of sex acts with other women my mom knows in exchange for them getting high. All of this is taking place while my siblings are there and aware of EVERYTHING. Continually fighting and both times 1 of my siblings found her overdosed and had to call paramedics to revive her. Well the past 3 months my youngest sister has been expressing to me how she is depressed and crys every day. I tried talking to her parents about it but they are both in denial because they dont want to accept the reality that they are to blame! Instead they blame her , saying its her age and her friends encouraging her to be that way. Which makes her feel worse , and lots of manipulation to the kids. My older sister has a job and boyfriend and is out of the house most of the time and is a little tougher with managing her emotions. But im concerned mostly for my younger sister because she is very emotionally fragile and easily influenced. She just admitted to cutting herself 2 weeks ago. And now after her finding my mom overdosed on the couch i feel like ENOUGH IS ENOUGH and someone has to take action!!!!! My mom is again saying "I will get help" but they continue to go back to there normal routine and nothing is changing. I cannot sit back any longer and let my sister suffer through this mess. I am 8 months pregnant with baby #3 on the way. Me and my childrens father have enough space for her but he is still on the fence about her staying with us. Ive had her at my house ever since the incident because i can see how badly her environment is effecting her and i feel like the best thing at least for right now is for her to be removed from the house. I proposed the idea to her staying with me and she does seem interested. I spoke to my mom about taking her until they can get their shit together and she agreed it will be good. I have yet to talk to her dad , he is the one i am concerned about because he has pride and its very hard to convince him that taking his child is going to help. Im going to present it like a temporary measure until we can think of long term options. Aka my mom going to rehab or them splitting up !! If he is not willing to allow this the only other option i can think to do is get DHS involved. I really do not wish to tear my family up this way but i feel like something drastic needs to happen in order for my sister to be healthy, a kid and focus on school. I really want to step in , but the only thing to consider is my boyfriends feelings. He is an only child with a normal family. He dont really understand the severity of all of this and feels like its not our place and we need to focus on our family first. Im trying to express my concerns about her well being , but im afraid that if shit hits the fan and it comes down to me taking her longer than originally intended, he may back out and start a fight with me over it. Im really torn but im set on the fact that my sister is in desperate need of help RIGHT NOW. And i cannot wait for every one to decide how they feel about things , someone needs to make a move !!!!!!!! Am i getting in over my head ? Am i doing the right thing? Does anyone have advice on how i should pursue getting my sister the help and stability she needs.