Well my advice is that you should have asked all of your questions to her that you had. Also what is your relationship with her? Is she supportive of your marriage? You have to be clear of her motive. Also where did she find out? Was it from your husband? If it’s from another source be careful.
You will have to ask your husband. Talk with him with what his sister has told you. Trust your instincts. Often times we know something is wrong way before it gets to infidelity. Be careful of what you share with your sister in law. Your objective is to only listen. Keep in mind that any information you share can and most likely will be shared with her brother and other family members as well.
I’m sorry this information is coming to you. As a wife myself regardless of where the information came from it would crush me. Try and again I say try not to overreact. The worst thing you could do is react totally off of emotions. Take your head with you and get to the bottom of this rumor.
I wish you well.