Are you paying rent to your mother? If you are paying rent, you should ask for a reduced rent since you are no longer getting your own room. Is there a basement? Another room on the main floor - something like an office or a den? Consider moving to one of these rooms.
If you are not paying rent, you are unfortunately going to have to abide by her rules. It's her house, and she can decide how she wants to use it. If you don't like the rules, you have the option of moving out. However, moving out, is not always feasible.
A 20-year old and a 12-year old sibling would rarely get along - given that you have so little in common at that point in your lives. However, you have to make an effort to do with what you have. Consider getting room-dividers that also function as bookshelves to increase privacy. Treat her like an adult, and agree on some ground rules. Instead of being bossy, try to work things out. "I like my privacy, and I am sure you do too, so how about we agree on a couple of things?" This could include not sharing clothes, defining closet space, working out a time of the day where each of you are given privacy in the room, etc. The key is to treat her as your equal instead of appearing to be entitled, simply because you are older.
I hope you understand that your new sibling (the baby) has to sleep somewhere. You have the option of sharing the room with the sibling, of course, but I would highly recommend against it. They sleep 16 hours a day, sure, but they wake up 10 times during the night too.