I think you have your own room now, but you want your sister's room. As you said "when she comes back she'll need a place to stay". Your mother wants your sister to find her room as she left it. Sounds reasonable to me.

i share a room with my brother

my mom wont let me get my own room until 2 years which is unfair because my sister is in college and when she comes back shell need a place to stay and the room has nobody in it can someone tell me how to convince her to let me have the room Thanx