I'm [25F] considering leaving my boyfriend [26M] of 6 years so that I can gain custody of my drug addicted, imprisoned mom's newborn baby (my sister) and he doesn't want to.

I just need some perspective. I am in a very happy relationship, even thinking about marriage soon. We have no major problems and I've always been happy with him. We both want to have kids, but not yet.
Well that all changed... my mom is addicted to meth and currently in prison because of it. Her boyfriend is 10x worse, if possible. He is not in jail but probably will be soon. They were both extremely abusive and made my life hell.
Well, my mom got pregnant. She just had the baby but was put into prison shortly after. The baby stays with the dad's family. I feel in my heart that I absolutely cannot allow that to happen. I don't want my sister to have to live the hell I did growing up. I want to gain custody of her and have her live with me. Both my mom and her boyfriend have agreed to allow me to have custody, because they'd rather be out doing drugs anyway.
I know it's not my job to fix my mom's problems, but this is a living baby who will suffer if I don't take her in.
My boyfriend and I have discussed this and so far, his answer is a firm "no". We've had many conversations about this and honestly it's becoming a deal breaker to me. He isn't obligated to take care of the child, this is MY choice and I understand his side, but that may mean we need to break up. He just said he doesn't want a baby straight out of school (he's about to graduate with his phd) and wants to focus on himself/us.
Two points I have considered and need to think more about...

1. Being a single mother is probably one of the hardest things a person can do. I have no kids of my own so I can't even imagine it. I know it's going to be hard, but I feel like I won't really know until it happens. I am a nurse and make plenty of money to support us.
   
2. It would tie me to my mom forever, basically. Even if she's not the legal guardian she is still my mom and I'm not entirely sure how to handle this.
   

I know this is probably above Dating.mobi's paygrade, but I just want to hear other opinions and things to consider before I make this choice.
Thank you.
tl;dr I want to gain custody of my mom's baby but my boyfriend doesn't and I'm considering leaving him.


Source.