I doubt that my husband and sister in law are having an affair

My husband and I got married 5 months back. My sister in law had issues with me from the very first day. She is 1.5 years younger to her brother. she used to sit in my room till 2 AM late night at the initial days of our marriage. she has gone to study now outside Delhi. But whenever she comes home, my husband tells me to go to my mother's Place and if not they she sleeps with us only. In fact the way my husband and my sister in law sister sits is weird as they always have a cosy Posture. please suggest how can the law help me? I have a doubt that they both have an affair.