"Possession"?

Eh, don't worry about her. I know a fair amount of people that know how to lucid dream at home. I can very safely state that none of them are possessed. And on here - well, how many people do you know that have been possessed? There are a lot of people on this forum, and of them, I think it's safe to say that we're all in complete control of our bodies.

One other thing - your mother sounds sorta... closed-minded. I know a few people like that as well. If the reason for your asking this question is that you want to prove to your mother that lucid dreaming is safe, forget it. Generally, closed-minded skeptics are going to stay close-minded skeptics, regardless of the information you present them. If, however, you simply want to prove this to yourself, then I can earnestly say that lucid dreaming is as safe as you make it. Don't worry about your mother. It is your mind, not hers.