Prison Yard Fights with Your In-Laws?

boy hitting pinata

So last night, my mother-in-law (we'll call her Maria) and her other son (not m husband) got into an argument.  Maria apparently called his wife (we'll call her Florence) on the phone, just to tell Florence off.    And from what I heard, it was a true verbal lashing - e.g. lazy b*@#ch  who doesn't do anything for her family, can't cook, clean, and overall worthless.  Yes, she used the word worthless.  When Florence's husband found out what his mother had done, he went up to talk to her.  It's his mother, of course.

Unfortunately, that's how the second fight started.  Maria began to apologize, but as soon as her son had something to say, she snapped again!  After issuing the second verbal whooping in a 24 hour period, she basically told her son that he was lucky she hadn't shanked Florence as she was cooking and had a knife nearby at the time!

Shank her!!!!  Are we on a prison yard?  Do I need to worry about crossing my mother-in-law too?  I know that I've had rocky relationships with my in-laws in the past, but this seems like a whole other level...  Today, I am looking for help before I become the next one on the hit list and get "shivved" in the back!

Does anyone have any tricks to rebuild relationships with their in-laws?  What have you dealt with and come back from?  Please share.