Infidelity and Addiction

I can understand your dilemma - being torn between possibly triggering your sister's addiction and wanting to be honest and protective of her.  It is always difficult to get involved in a couple's relationship.  What happens inside a relationship is difficult to know from the outside no matter how close you are to someone in the relationship.  I would suggest holding off telling her.  Give the situation some time to see what happens between your sister and her husband.  Is it possible to talk to your brother-in-law about what you know and your dilemma?  It might be insightful and helpful to hear what he has to say and where he is coming from.  Without all the facts and knowing all that goes on in a relationship, telling your sister may trigger your sister's addiction and she may turn against you for affecting her and her husband's relationship.  You are not being disloyal by not telling her, you are being cautious and careful.