My mother adopted a child and she won't leave me alone, what can I do?

I'm 16, and was an only child...but then my mother adopted a little girl from the Ukraine, well she's 14 but she's small/thin for her age and has Cerebral Palsy so can't walk. She suffered a lot of trauma in her past. She got attached to my mother fast...and me too for some reason, and now she wants to do everything with me and even wants to sleep next to me at night in bed. She asks me to do things with her all the time, and wants to be everywhere with me, and then she looks like she's going to cry whenever I give any slight indication that I don't feel like it. How can I get her to stop wanting to be near me constantly?...I'm really not used to this after being after having the house and my mother all to myself for so long.