Parents and husband are fighting

I just need some advice really. Me and my husband live with my parents on a farm. We have our own house but still live next to them. I also work with my family so it’s not ideal but we have always made it work. My husband will not take disrespect from my mum, my mum is a hot head and flys off the handle at the smallest things. The last straw for my husband was because my mum was swearing in front of my step daughter his first daughter from a previous marriage and she told my mum there was a child here and my mum looked her dead in the eyes and told her she didn’t care. Now that’s what’s pissed him off, not the swearing as if she had said sorry I know she would have been okay but it was the lack of empathy for a 9 year old little girl. I completely agree with my husband and will back him until I can’t speak anymore but I’m just stuck in the middle all the time. We can’t afford to move out as we are tied into this property but ever since she has done this she has argued with me everyday saying that his feelings don’t matter and that no one else thinks that she was wrong. My husband is miserable and is drinking his pain away, which is a huge no from me as he only has one kidney! This is causing us to argue and I’m just defeated, I honestly just want to floor to swallow me up and I wouldn’t have to deal with this anymore. I adore my husband and it hurts me to see him this way, but I just don’t know what to do. My mum will not admit she was wrong and thats al he wants to hear from her. We are walking round like we are strangers in our own family....