Brother in law

So my sister and her husband and their five month old baby are living with us for a little bit because they had to move with little notice because the house they were renting got sold. My husband and I let them move in with us, so they could save money to buy a house. Both of us were reluctant because my brother in law has a drug and alcohol problem. They moved in and I have been keeping track of our narcotic pain meds. Today we were two short, so we confronted him and told him he has to move out by the end of the week, my sister and the baby can stay, but he has to leave. I feel so bad that my sister had to go through this, because he has had this problem for a long time, and she always stays be his side. How should I support her or what should a say to her? This isn't the first time he has stolen from us or from our other family members. Help! please!