Is it okay to sleep with my sister topless?

She's only 10 years old and I'm 17 and we've been doing it for 2 and a half years now. My mom told her that she would have to learn not to sleep with her so one night, she just slipped into my bed. But I always sleep shirtless with my boobs hanging out! its just too hot to wear a shirt! She always comes in to my bed around 11 at night. She likes to cuddle real close to me, and i've got a real comfy area in my cleavage that she always lays her head in... We live in a small community and she's home schooled so being made fun of by her friends isn't a big deal or anything, she just hangs out with me, our other sister, and sometimes our older brother. Is this wierd? Should she learn to stop? If not now, when should she stop?