I think what is awful is that your parents do NOT respect your answer. No means no, if they want to see BOTH of your children when they return from vacation, that might be fine, in their home for a quick overnight, but this constant harassment towards you is ridicules.
Using $$ to bribe you to change your mind is just insulting. And I would tell them so when they return. You are the adult, the parent, not some irresponsible kid.