i agree with everything that whatyoumakeme said, although i would like to add that a 3 1/2 year old should not still be taking a bottle. they need to find something else to help soothe her to sleep.
I'm a retired dental hygenist and sadly this happens to often. In my personal opinion, its a form of child abuse and many in my field would agree. I have seen 4 year old in my chair that looked like they just ate oreos. Its very painful for the child and breaks my heart to see them suffer. She is so young to have lost 4 teeth, she won't get the adult ones untill 6-7 years old. Its also been know to affect the childs selfesteem, especially when they start school. Baby teeth are placement holders for the perminate ones, if she doesn't have them she will most likely need braces to straighten them later on. And like I said, its painful. Your mom needs to be educated on baby bottle tooth decay. Babies and children should NEVER be sent to bed with a bottle! Not water, juice, coke, or even milk. You should suggest to your mom that you sister see a pediatric dentist. Kids start going anywhere from 3-5 years old on their first visit. She is due and maybe they can give her some tips and make sure your sister isn't in any pain. That way she won't feel attacked or like your calling her a bad mother. Let the dental people talk to her. I hope this helps. Your right to be conserned, I'm proud that you knew to look for that. Let me know what happens or look me up if you need to talk! Take care!