There will be a back story here. I have also noticed that the OP won't say a word against her DS, but surely he shouldn't take some of the blame, particularly over the lack of tea offered. (That to my mind is plain rude of both of them, imo.)
I will admit that I have a difficult relationship with my MIL. She's a lovely grandma to our DDs and they love having her to stay so I do my best to get on with her, but it's a struggle to get on with her and I can't put my finger on exactly why, as has been said by a PP.
My DH was 37 when we married and he hadn't been married before. He was therefore very close to his DM and DF, who sadly died in a car crash the same year we got married.
My MIL has always been very full-on and gushing and very tactile, which is something I find very difficult to cope with. She wants so much communication by telephone and has always phoned a lot. When she's here for significant periods, I do disappear to do my own thing (but she's well entertained by my DH and DDs so I don't really think that's an issue).
It was also tricky because she really wanted me to call her 'mum', as she did her MIL, but I always refused. I think she was hoping for a closer relationship than I was prepared to have with her, and tried too hard. We've found our way, though, and now we communicate by email which suits me much better.
I'm seeing some similarities with what the OP is saying, though the lack of decent manners is surprising in two adults. But I do think the DIL has been made into the scapegoat when the DS should 'grow a pair' as it were.
As for SS, I won't comment as that seems complicated. I know of a situation where an elderly mother nearly called SS on her son and daughter-in-law, because of concerns her grandson was being neglected, but didn't so as not to end up NC with them. If there has been DV in the past, there's a lot of back story quite obviously.