How can I adopt my sister?

im 23 years old and since the first day I remember my family was a wreck.my parents were always fighting and my dad was very abusive.when I turned 17 my mom filed for divorce and me and her filed a complaint against my dad and in court in was found guilty and they made a file for him saying he has mental problems. after that my mom started verbally abusing me and kicking me out of the house at late night hours.after 3 years I got tired of it.and moved out.after 8 monthes she asked me to come back and we fix everything but when I moved back it all just started again.right now im 23 and on my own I have stopped talking to her for 2 years now.

but the real issue now is that shes doing the same thing to my 15 yr old sister.my mom has remarried again and the guy is a .my mom is verbally abusing my sister and tries to kick her out of the house but cause she has full custody and my sister is under 18 she cant kick her out.but everytime I c my sister she looks more depresses than last time.she wants to get out of the house.my mom is moving to the US with the man and she wants to take my sister but my sister told me that she doesnt want to go.my mom and her husband are really hurting her and I feel like in a little bit time shes gonna crack and do something to herself. I know she can talk to a social worker but we dont want my mom find out.I just want a way to get her outta there and this counsellor I talked to said I have a option of adopting her.

what can I do?