Why do you want to separate a child from her only parent left? Also, It’s not your baby that’s his baby, his child. And are you even sure your niece is actually getting neglected or treated badly?
Your BIL being the father of your niece is her natural guardian. Your case is weak in front of his. Unless it’s proven that he is abusive and your niece’s life is in danger, the likelihood of you getting her custody is almost zero.