Busybird you have my sympathies. I too hate get togethers with the in laws, and avoided them for years by feigning illness, prior engagements, even bookintg a last minute holiday to avoid spending Xmas with them. However OH asked me to attend a 70th birthday party for MIL and I reluctantly went. Shall we say I don't want to repeat the experience any time soon, but it was OK.
Don't listen to them. It's what OH thinks about you that is important and he likes you the way you are. Let the children stay at home, book a taxi for 10pm (say you need an early night as you have a busy day the next day ferrying the children around, or you have to get back for the babysitter, and OH will need a good night's sleep as he has to drive to the airport) Alternatively take the children and use them as the excuse to leave early!
And as Ygerna said, remind OH that you are his priority now he is married. Men find it difficult to escape from their mothers, many still think they have to justify every action to them. He does need to tell his sister and mother to keep their opinions to themselves, particularly if they are spiteful, problem is he may not feel brave enough!
There's always Rescue Remedy too!