Oh holy Crap. You really need to take a bit of a step back and overview.
Step families is a bizarre thing. You have this person who you love. But you also have to build entirely separate relationship with these other human beings you may not have natural sympathy with, you may not even like. The same is true whatever age the children are.
What's more these people are and should be more important to the person you love despite the fact that really we all want to be number one to a partner.
A good man won't let you alter the dynamic between him and his kids. But that also means a good man won't ever put you first and actually, if you want him to be a good man, you shouldn't want him too either.
Not everyone can, or should have, to be that self sacrificing
At a year and a half living together your really still only learning to be with each other. You've only really gave one whole years cycle to work out how family dynamics work. Your at the stage where you are wanting to stamp your own customs and practices on the relationship. In an already established family that really isn't how it's ever going to work.
I dont think many people explicitly make these things clear when you enter into a relationship with a preexisting family.
Use this time before the wedding to take stock. Delay if necessary. You are still the baby in terms of your relationship with this family
Have you discussed things like wills and divorce?