It isn't very clear whether your niece's mom is letting her live with the 21-year-old or if she's letting the 21-year-old live with them, but regardless, it's not good. If she's letting the 13-year-old live with the 21-year-old that could be considered something like "failure to protect" and definitely some kind of child abuse/neglect. This is assuming that the 21-year-old is the 13-year-old's boyfriend. (Not sure if the 21-year-old is mom's boyfriend or the 13-year-old's.)
If there's any sort of sexual behavior between the 13-year-old and the 21-year-old, then he would at least (and maybe mom) be in some trouble. That looks like sexual abuse of some kind.
Lastly, yes, the mom would be guilty of neglect if she isn't making her 13-year-old go to school. Consider reporting all this to CPS. Something doesn't seem quite right, and you don't want your niece to get hurt. Call us if you need the number for your local Child Protective Services.
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I think you need to call the cops or do something about it yourself because when you dont go to school thats a way big problem. She's only 13 also thats bad real bad she's probably in the 8th grade wooh thats a real situation
first the all i need you to call the police and tell them whats going on and that's not good for a 13 yr. old to date a 21yr. old man and i need you to tell her she can get hart by a 21yr. old man and tell her that she need you break up with him and that is sanitary rape