My sister in law is on drugs and leaves home for extended periods leaving her 15 year old son at home with her boyfriend. She leaves for weeks at a time and her boyfriend is now leaving her. Her son is left alone, and my husband (her brother) wants to take custody of him. How do we go about doing this?

Hire an attorney.

He will file the appropriate petitions, you will be fighting to prove the mother unfit.

An alternative is getting your local Children's Services agency involved. You can approach them that you are concerned that a 15 year old is being left alone for long periods and that you would like to foster him.