Left my son and his friend at my mother-in-law's. We live with my husband's brother and his mom in t…

Left my son and his friend at my mother-in-law's. We live with my husband's brother and his mom in the same house; we're building our own, and only the renovation remains. The point isn't that you'll start writing that we need to live separately—we know that perfectly well. I picked up my son from practice, and his friend stayed too because his mom asked him to; they were running late (the boys are 8). I left immediately with my daughter for dance class while my husband worked on our house. When I arrived home an hour later, my son said his husband's brother had locked them in separate rooms and wouldn't let them out for 20 minutes. After my son asked to go out, the brother hit him on the cheek and said, 'If you step out, I'll kill you.' My husband, hearing this, told his mother: 'What the hell are you raising your hand against my son? Who exactly are you? Either you talk to him, or if you do talk, it will be very bad!' At that, the mother-in-law cried very hard (I wasn't home at the time). When I arrived, she wouldn't speak to me at all—not even after two days of no contact. When his brother came over, I heard her in the kitchen asking him, 'Did you hit him on the cheek?' He replied, 'Yes, he deserves even more; why don't I have the right?' I said nothing to either of them. She just stopped communicating with me. Even today, my son asked her to pour some water, and she pretended not to hear. It makes me feel so uneasy… I can't understand where I fit into this? What did I do to her that she won't talk to me anymore? How would you react if your mother-in-law, while living in the same house, refused to speak to you even though you didn't do anything to her?