C.M.
I'm not sure that there is a way to confront your brother. However you can always try, but like you said if he takes off then the only real females your neice knows will be gone. If you feel it gets to be a big enough problem then consider talking to dhs. they can keep it confidential and if your niece is not getting taken care of like she is suppose to be then the best place for her to be is out of the home, but not with her mother either, because that seems to be a little too much responsibilty for a young girl like that. I have a step daughter who came to us because she wasn't being taken care of by her mother and not having clothing that fit her and being neglected really effected her in many ways. not only at home but with the children at school also.
well good luck and i hope you can help her.