I'm not going to allow my daughter to go on a family trip with her dad and stepmother
Why won't you allow your daughter to go on a trip with her father and stepmother?
I don't think her stepmother is a good influence on her.
It's understandable that you would be concerned with your daughter going on a trip with someone you have doubts about. As a parent, you have the right to decide who your daughter goes on a trip with. Talk to your ex-husband and explain your concerns.
I already had a big talk with them. My reasons are solid and they will have to understand that.
I'm sure they'll understand. Anyway you're her mother so of course you have every right to decide for your daughter.