Hi Greeboclovis.
I am not sure how I would approach this if it were me but thinking about it I think that my dh would be angry with the idea too. He would say that I am his wife and I will attend as his wife.
But that is him. How do you feel about his idea of being pushed aside?
Thing about stepping is that you should not expect too much from it. The children do not have to love you, and you do not have to struggle and pretend to love them as your own.
As long as there is respect between you then you should be fine. Respect takes a while to develop, especially in children. It may never come about but yr dh feels he has a responsibility for your happiness too.
BM is doing her best to show that she is insecure and unhappy that her children's father is married. The children's ages (IMO) are at a very vulnerable stage. OF course they will listen to their mother, they live with her. It will eventually wear them down and they will see the situation as it is, from their own point of view... eventually.
Bad mouthing BMs is far too common but in some cases there is just cause. Bad mouthing steps is a no no in my book. They have such a difficult time trying to please everyone and putting themselves last, they sometimes feel lonely and left out of the family unit. (Not intentionally tho).
It is important to step carefully to begin with. You may not be the childrens' mother but you are an adult who deserves the chance to earn respect.
I hate being a step parent at times. If someone had told me what it would be like, I would have said no thanks. Much as I love dh and all the children, stepping is not my favourite occupation.
Do you have your own bio children? Are you planning children with dh? - forgive this personal question but was dh divorced when you met him?
You need to keep your chin up and be strong because it is not going to be easy by the sounds of it.
You will beat BM's attempts if you remain calm and non confrontational. Not easy, I know.
{{{{HUGS}}}} and hope that you don't worry too much - try to enjoy your trip. You are there as dh's wife - not his children's mother.