Adopting my younger sister- what to do?

I have no idea where to post, so I'm sorry if this is the wrong place. Backstory: My parents were married for 22(ish) years. I was born in 1994, I turn 21 this month. My brother was born in 1996, he is 18 now. My sister was born in 2003, she is 12 in September. Both my biological mother and father are alcoholics. I stopped contact with my mom a long time ago, last time I heard anything from her was in May. When I got married, and she showed up completely trashed on listerine and percocets. My dad has re-married, and my step mom is wonderful. She has her own daughter, who is my age. She is practically supporting my dad and brother/sister, seeing as my alcoholic father lost his job. And it's only a matter of time until he loses the house. He doesn't want to get better. Child protective services have been involved a couple times in the past. I remember when my sister had to stay with my grandma for 90 days, and she cried and cried that she wanted to go home with me. At the time my boyfriend and I hadn't married yet, and we have a tiny studio apartment. No room for her no matter how bad we wanted to try. Now, we're looking into a bigger apartment. One or two bedrooms, enough room for her, too. If child protective services gets involved again, she's going to foster care. She doesn't have anywhere to go except for me. Grandma already adopted two boys from other family, she can't stay with her mom (no questions asked.) and has no family locally that can take her. I just have no idea what to do in terms of insurance (I don't have health insurance.. was covered by my dad, until he lost his job.) and what we would have to do in order to have custody of her and be her legal guardians. My husband and I both work, and currently we're literally right down the street from her middle school. So she can walk to and from when school starts. If we move, we'll be looking to get close to another school. Has anyone done anything like this before? Any tips? This isn't something that 100% is planned, more or less "it's only a matter of time" so we're trying to be prepared to take her in at any given time. My dad has no idea. If she comes to stay with me, I don't want her to see her mom anymore. She's toxic. She's constantly wasted, and quite honestly I worry for my sister when I hear she's with my mom. Would it be terribly detrimental to my sister if I stopped contact with her mom? She's mature for her age, so maybe she could understand one day. It's just a whole bunch at one time. I don't know where to start.