Sizz - So, you don`t know that your brother in law is cheating. And you are clearly ok if your brother in law tells your family that you see SPs. You know, he might find what you`re doing, regardless of whether or not you`re single, to be unacceptable. So sure, hire a private investigator, gather some bogus evidence and rat him out to your sister, your parents, his parents, his employer.
Get serious, do you think that`s your role here. Has your sister asked you to do this, I doubt it. Unless you have a reason to believe he is spending his money on SPs, you`re only coming to this conclusion because you do it yourself. Why don`t you try this - talk to him like a man. See if he needs some help. Offer your assistance without all the judgement. Then, mind your own business. The last thing anyone needs in a marriage is for a relative to get deeply involved in their personal matters. If there was obviously physical or psychological abuse, sure you have to do it. But from what you have described, this is not the case.
Back up off of there. Let them be. Offer a hand and maybe it will be accepted. Otherwise, leave them alone.