Maybe you could redecorate her room , it doesn,t have to run to great expense just a few things to make it special and different . Sometimes this is a cry for help , have you asked her if she is afraid of something or if she is being bullied at school? . Another idea is to let her invite a friend for a sleep over, sometimes this can help the transitional stage of child to teenager as they feel more grownup and hopefully she will want the privacy and her own space too. I hope this helps. Good luck.
Best wishes
Angie
Well, Jade, I think you should look at this as a blessing. Your sisters kind of odd behavior has kept your mom from bringing men over to the house to sleep with, which puts you and your sister at significant risk for at least emotional abuse, if not worse.
In 5 more years your sister will be up and out and your mother will then have all the freedom in the world to have whatever relationships.
Meanwhile, you and she have been spared the chaos of lovers in and out of your mom's life, step brothers and sisters, ex's, new pregnancies, etc. That's quite a gift, really.