I want to help my sister, but I'm at a loss...

I don't know where to post it. Probably posting this in various places.

*Extremely long post. Please read it all.*

Back story:

When my brother was 10 and I was 14, we got taken away from our mother because she is an addict. She was an alcoholic and a pill addict. She got crazy and did so much crazy things. Her husband watched all the shit that went down and still stood by her side and protected her instead of us, saying we were lying and my grandma was evil all throughout the court process. We got placed with our grandmother, my mother was deemed unfit, and my father signed his rights away so their rights were severed. (They weren't together by the way. My dad left when I was 4.)

I hadn't spoken to my mom in the year following. I was angry and hurt. Well, she overdosed and almost died, and I thought that was the universe telling me to get over my anger and make amends with everything before she died. I didn't want to be angry with her and have unresolved issues with no outlet. I told her to clean up her act if she wanted a relationship with me so she did. We had our relationship for two years when she told me she was pregnant with my sister. I told her flat out that she better not fuck this up like she did with my brother and I, because I would do anything it takes to protect this child and choose her over my mother. She promised she wouldn't. I believed her like the idiot I am. She was sober for a little while after that or so I thought... My sister is now almost three years old. My mom says she was sober throughout everything and this and that. She's a liar. I found out recently that not only did she have pills and weed in her system when she had my sister, but so did my sister. CPS was called on her shortly after the results came back and my mom told me that it was a "routine checkup since she just had a child". Lie. My sister doesn't talk besides two words. She just barely started walking. They keep her in her crib the entire time. It's so frustrating... I really started noticing my mom's drug addiction was more prominent about a year or so ago. I denied it, didn't want to believe it. I didn't want to have to hurt my mom by trying to take her baby away... But anyway, CPS ended up getting called and my mom's husband called me, blamed me, screamed at me, and basically I'm not allowed to see my sister. CPS didn't do anything.

Present situation:

Some shit has happened that I can no longer deny or be selfish about. I can't let her ruin my sister, but I don't know what to do... This hurts so bad and I'm crying as I type this. Please, help me. Is there anything I can do? Anyway I can at least force my mom to let me see my sister? I can't take my sister from her and have her placed with me right now for various reasons, but I believe my grandma could take her in and I'd be helping out. It's just getting there that's the issue... CPS has been called three times, I believe and nothing has been done. My mom can stay sober as long as she has to to do whatever she needs to do, use whoever she has to, and manipulate whoever she has to. Her husband isn't much better seeing as he's pilled out, smokes weed (not bashing it, but when you aren't taking care of your child, there's an issue), works, makes excuses for her behavior, puts my sister at risk, etc. The difference is, my mom has an extensive record, he doesn't have so much as a speeding ticket. Any advice is appreciated. Please...