my sister is making me angry

This is my first post to this forum. I'm feeling torn about my situation with my family. I live on my own and my little sister still lives at home with my mom. My sister is old enough to move out on her own but she won't do it. My mom wants to have her in the house with her because she gets lonely and wants a companion. I think the relationship is very unhealthy for both of them. My sister can't grow up and leave the nest and my mother is lonely and she wants my sister to depend on her, my mom babies her and my sister likes it -- though my sister would never admit it, nor do i feel comfortable telling her.
So my sister has a bit of a temper. When we were little she used to throw these temper tantrums all the time. Some time during her tantrums she would bite me. I think in some ways her tantrums have progressed so though she doesn't bite anymore as an adult she does do other things. I am afraid to fight with her because she has a hard time controlling her anger and she goes from 0 - 100 in a second. She has almost hit me many times, i know she wants to. She gets red in the face and she gets in my face to the point where our noses are practically touching and from her yelling i can feel the spit hit my face. It is so scary and i try to stand as still as possible and not say anything because i think one little peep would actually make her hit me. She will eventually calm down but i hate any kind of argument at all with her. What calms her down is usually storming off and/or wrecking stuff around the house (usually items that don't belong to her).
What i worry about most is the way she treats my mom. My mom was emotionally abusive, manipulator, and compulsive liar to us growing up so she has her problems too but i do feel that my sister is physically abusive to my mom. And now that she is bigger than my mom she can over power her. My mom has told me stories of my sister pulling her hair and pushing her to the floor and kicking her. And just yesterday my mom was whispering to me that before i came over my sister got so angry with my mom that she wrapped her hands around my mom's neck. And then my sister walked in so i didn't find out the whole story. Now i'm sitting at my house just so pissed right now and so confused. I don't know what i should do. i love my sister and i love my mom but i don't want to see my sister do this anymore. I also don't want her to get in trouble. I'm just afraid of what might happen if i did tell someone. She blames other people for the way she behaves, she says, "You made me do this."
I didn't know if i should also post in the Bully Forum or another one because i do think she is a bully too. She tries to intimidate people to make them do what she wants (mostly her family). And people do it and i think mostly because she is the baby in the family - a 23 year old baby.

Hi, brooklyn! You're right, your sister shouldn't be abusing your mom even though your mom was abusive to the two of you. It's not a healthy situation at all. I guess your sis might stay in this situation because she feels like she needs revenge and to have the control she didn't have as a kid.
If she hits you, you can hit her back or call the police. I think you'll have to make your mother see that this is bad for both of them before you can solve anything. If she doesn't want your sister to leave, there's not much you can do. You should try to reason with her. They'll have to take responsibility for their own lives at some point and you have to live yours, you can't worry about them all the time.

Thank you for the response.
I have talked to my mom about pushing my little sister out of the nest and my mom agrees but i know she won't do it. So i guess the only thing for me to do is to butt out.

You're still there for your mom and I'm sure it means a lot to her. Who knows, maybe one of them will change their mind.