My in-laws falsely accused my younger brother of stealing

We live around 150 kilometers from my in-laws. Since my father-in-law’s health deteriorated because of age, we have started to drop by more often.

A couple of months ago, my husband, my mother, my younger brother, and I visited my in-laws’ place and stayed overnight.

My sister-in-law called my husband a few days after that visit and told him that my younger brother stole several million dong (VND1 million equals $42) in cash she had left in her wallet.

She said no one other than my mother and younger brother dropped by, and my younger brother took a trip with his friend around that time so he might have needed money. All pointed to her guess that he must have stolen it.

I felt upset because of her accusation, but I could not say anything to protect my brother. I am sure that my brother never steals.

My husband did not know what to do either, so he stayed silent. I told him that I would not let anyone in my family visit his family anymore.

My in-laws have not talked to me even once since the event, even though I have always gotten on well with them.

If I do not say anything, I may lose the previously good relationship with my in-laws. But I do not have any proof either.

What should I do?