Well, MIL and DH's stepfather, but I'll call him SFIL. I have always been polite and pleasant to them, but have just had enough tbh.
When DH was growing up, MIL used to leave DH and his brother, aged 5 and 6, locked outside of their house so she could have an affair with SFIL. She also allowed SFIL to hit DH and his brother with a piece of 2 x 2. (wood) The first time I met MIL, when DH went to the toilet she told me she knew his first marriage wouldn't last as his first wife was 'too good for him'. 
Luckily they live abroad, but whenever they come to stay they are very rude and treat the place like a hotel. (recently posted about this) They never say please or thankyou and never do anything for themselves, despite me making it clear they are welcome to get their own drinks etc. They always ask me to do it. When they stayed at the same time as my Mum last year they treated her like a servant. In the end I told her to stop doing things for them, but she wanted to to keep the peace. This time when they stayed SFIL joked about how if my Mum was here she would have just been moaning about having to do things for him. 
I have three sons from my first marriage. My youngest DS, who is nearly 9, has Downs Syndrome. SFIL has openly called him 'backward' before now and treats him like an idiot in spite of myself and DH explaining to FIL that DS understands everything pretty much, but his speech is very delayed. My in laws ignore my DS's birthdays, not even a card. (ironic seeing as SFIL refuses to ever have anything to do with his own DC apparently as they forgot his Birthday once!) I am not even sure they know when they are, and don't usually get them anything at Christmas whilst openly giving DD money. SFIL also subtley bullies DS2, (11, young for age, waiting for a possible ADHD diagnosis) IMO, things such as offering to shake his hand, but gripping it really tightly so it hurts him. He also makes lots of nasty comments to DS2, such as calling him stupid etc. He does it in a 'jokey' way, but it's not funny. I intervene as much as possible when this happens, whilst trying not to be rude.
I'm absolutely sick of them tbh. I feel they look down on me and my family. I have told DH that for future visits they will have to stay in a hotel and that I want as little as possible to do with them.
AIBU?