Is uncle of a woman (sister’s husband) mahram or a non-mahram to her?

Assalamu Alaikkum! Is uncle of a woman (sister’s husband) mahram or a non-mahram to her ? The situation is : I got a controversial argument from my wife. i.e. One day she slept with her uncle(her sister’s husband)in the same bed during day time in the absence of her sister. During the night time she is sleeping with the same person along with her sister. Unfortunately Im out of town.(Allah may know all). This incident came to my knowledge and I discussed about this with my wife, but she argued me that her uncle is like “her father” so nothing wrong in it. Even she refused to wear hijab. But, I feel like it is not a moral. What is the solution for this in light of Islam. Thanks