Well, I feel like I am in a similar situation, except that I would never do the things your DIL has done, I am more like the DIL a few messages before that stated that she talked to her husband and did not lash out at his mother.
But I do feel that the tension between my husband's mother and I is very straining for him. I tried everything in the book to get her to like me. I bought a house near his mother and he came home on a 2 week leave from Iraq to do the move, but while I was up here visiting to see the house I stayed with them and I was polite, and even did the dishes after she cooked. I feel like I've been forced in the position I am in. I don't know where her hostility came from, but I know his mother and sister are gossips and some stuff was started while he lived with me and his sister stayed for a month, 2 weeks he was even gone on AT for army. I kept his sister from running around with a black man that was like 15-20 years older than her, she was 15-16 years old, so I think that pissed her off. Also, I am very passionate so I get angry easily and she had seen me go off, which I had reactions to narcotics for my militay injury that was making me very violent. There are some things I don't even like about myself, but he married me knowing everything and they should not meddle in our business. But I cry and I've lost my mind with this crap, especially when I moved 6 hours away from all my friends and family. My parents are in the merchant marines and are not easily contacted. I had no one to talk to and he was in Iraq, they attacked me non stop...especially his aunt!
I would do anything to be friends with his mother, but if I was not his wife and she was just some person I bumped in to in town, I would not be her friend. She is controlling, and manipulative, and hurtful, and still continues her life as if she was in highschool. His mother, sister and aunt are outragious in public. I've never seen such horribly mannered people. His sister yelling out to people they are fat, burping and farting in public, shes a pretty girl I dont know what's wrong with her. His mother says "what The **** you looking at " when men check her out. His aunt is just insane all the time! His aunt doesn't want to be called fat but she is the first to call someone else it. My grandmother was a southern bell, and I came from money and I was even disowned at 13 years old for being upset about eatting at a smoke filled retaurant in Mexico by my grandmother. So seeing people like his family blows me away and is embarrassing but I just tell his sister to calm down and laugh to ease my disgust. Her daughter and her are so much alike its scary. I think my MIL is having a hard time adjusting to her aging, she is skinny, decent pretty, and obsessed with her appearance. Other than some things we have in common, she and I are very different when it comes to life. I see her as one of those highschool girls that ran around with the perfect hair and after guys. Me, in highschool, I was a tomboy, I played male varsity football, was in honor and college prep classes, JROTC, BAND, and only seriously dated 3 guys my highschool years, HER SON INCLUDED! My first boyfriend was an obsession from JR High, Gary was the real thing, and the next guy I dated over a year and maybe saw 4 times because his mother thought I wasnt CHRISTIAN enough. With the later I do appreciate that my MIL is mine because the last guys mom was a nightmare.
But I do believe the comment someone made is that mother son relationships are pretty sick. My own mother on the other hand is awesome, she has taken in my brothers soon to be ex wife and treated her like her own, until my SIL wrecked their car with a lawsuit pending, and then mom is really good to my brothers new girlfriend. My mother has always been reserved, not in the old fashion sense, but in the sense that she doesnt like confict and knows there are no winners when others meddle in other peoples marriage.
I did not even know my mother and father hated my first husband, even when I was crying saying I was coming home, she never said a bad thing, just said the porch lights on.
I just don't know what to do.