Hi YA,
Who btw invited this toxic pair to visit?. Your H?. How long are they in your house for this time around for them to lord it over you?
"They have no idea I feel this way though. And the DC are oblivious to their toxicity - they like to see them because they bring them shit. LOADS of it".
You have a real problem on your hands, one that is not solved by playing bs bingo and saying aloud to them, "did you mean to be so rude?" and so forth. The "normal" rules of engaging with familial relations simply do not work when it comes to inherently dysfunctional families like your H's. The rule book infact goes out the window.
Re this comment to which you responded yes:-
"Is he one of those "I cannot possibly say no to mummy, she might get upset" knobheads?"
I was not at all surprised. This is correct on one level but it goes deeper than that. Far deeper.
Your H is also mired in FOG - fear, obligation, guilt when it comes to his parents because he has had a lifetime of such conditioning from them which is very hard to break. He is only hurting his own self as well as his family here by acting as he does towards and around them. He like many people of such toxic parents, keeps coming back for more and hopes still that they will change and like him. That will never happen though.
Unfortunately as well your children are aware of their toxicity because they see how you and their dad react towards their grandparents. These people insult you and by turn them. You cannot respond because you feel you cannot (your DH gets upset) and your DH will not respond to them due to inbuilt conditioning to accept their mad behaviours. He is really hurting his own self and his own family here by acting as he has done. Your children will not thank you for putting up with them out of some societal convention and it would not surprise me either if they played one sibling off against the other or showed overt favouritism towards one child. Your DH is still wanting their approval as well, approval which they will never give btw.
All you can do for yourself ultimately is reassert and re-assess your own boundaries and protect them from their malign influences. These people also make for being deplorable grandparents as well as ILs.
Also you likely come from a family which is completely unlike your DHs one; i.e functional and emotionally healthy so have wished to be reasonable and nice. That worked against you. These people as well I daresay have never apologised nor have accepted any responsibility for their actions.
Your boundaries re them are practically non existent so these need to be raised as of now for example take the children out with you and do not engage with them on any level when they arrive. Your DH needs to be aware that you are not prepared to be your ILs sacrificial lamb to their narcissist needs any longer. He can have a relationship with them if he so chooses but it does not follow that you and by turn the children have to. His primary loyalty after all should be to you now as his wife, not his dysfunctional and toxic parents. Remind him as well that if they cannot or will not behave they do not get to see you or their grandchildren, end of.
I would also suggest you read Toxic Inlaws written by Susan Forward and you may also want to post on the "well we took you to stately homes" thread.