Why do you have to go at all, you have a very good idea of how this is going to play out. What consequences are there for you if you do not go?.
It also does not have to be at all openly hostile for such an occasion to be absolutely awful.
Fear, obligation and guilt are but three of many damaging legacies left to now adult children like yourself oceanrose. Fear is driving this as well I think in that you perhaps feel that you cannot not attend this event.
Honestly I would not at all go to this event, you are the scapegoat for their inherent ills and will be treated as such the whole time you are there.
It will do your children no favours at all, particularly your eldest who is all too aware anyway of what your family of origin are like, to see their mother keep being so disrespected by their grandparents. You're simply showing them that this abusive treatment of you is still acceptable to you on some level. It will further damage them and you by exposing any of you to this on any level. You are showing too your own children that you do not at all value yourself.
You have no car either so simply cannot walk out and drive off.
People like your parents and siblings (who are just glad that you are copping the fallout and not them) are NOT going to change, you can only change how you react to them. You perhaps keep going back as well in the hopes they will change, properly apologise or even accept any responsibility for their actions; such people do none of these things. Surround yourself instead with emotionally healthy people, not those who are happy to do you and in turn your own children down. You do not need them but they need you to keep the scapegoat dynamic going. You do not have to play their game any more.
I would suggest you read "Toxic Parents" written by Susan Forward as well as posting on the "well we took you to Stately Homes" thread on these pages.