Wait till they do fall out and your gums start bleeding badly, if she still doesn't take you then call CPS (Child Protective Services) on her *** cause shes a deadbeat mom and deserves some jail time for neglecting her kids' gums. Or you could do what Anonymous said and talk to her about it. Tell her you need your teeth cleaned and while your at it ask her what kind of parent she is LOL! No seriously dude you gotta see a dentist.
Update: Ok then don't call CPS - actually, I was kidding about that. I thought you would've got that. Guess it wasn't funny.. ya, don't do that. Anyway.. it sounds like your mom is what's called a procrastinator and that doesn't make her a bad person or parent or anything, but you really gotta be assertive and get her attention, and get her to take you to the dentist because 7 years is way too long to neglect your gums. A nice set of teeth goes a long way, trust me on that one. As for your mom being violent, I can't really touch on that all I can say is I hope she's not too out of control and that you're not living in fear or unhappy or anything like that. Basically, all the best to you, have a good one, and, seriously dude, find some way to get to a dentist before you turn 18. Meanwhile, brush and floss daily.