I am going to call my brother in law an alcoholic.
Are you concerned that your brother-in-law may be an alcoholic?
Yes, I'm concerned that he would pass his addiction to my sister since he frequently drinks in her house.
If your brother in law is an alcoholic, then this is a disease that needs the whole family to address it. You should privately speak with your sister before confronting the situation to find out if anything is being done to treat your brother in law. Be patient and helpful when your family needs you.
FUCK, you don't know how hard it is to talk to my sister. It's her fault for tying to change the situation when she can't even do it by herself.
Perhaps talk to his friends or other family members, then. It's a delicate topic.