Your sister in law is not immediate family and is more likely to beleive you, but the risk is for your brother who is so close to the situation he may not want to beleive it. The worse case scenerio is if your brother rebutes your confession and if so, what would he likely do and how will it affect you? Best case scenerio is if your brother offers you compassion. Could you tell your sister in law and not your brother? She may pave the way to what you feel you need to do. In the end, it's all about you and your wellbeing. I beleive you and send a prayer of healing and strength for you.