advice for a prospective step parent of adult children

[Justus]

I posted a question on another page and a kind person directed me to the step-parents - here's the link to my problem - www.mumsnet.com/Talk/parenting/1649344-How-do-I-stop-my-attitude-to-his-adult-children-ruining-relationship
Any advice would be gratefully received. I love this guy but we always seem to have difficulties over our children. It feels like we will always have to take a back seat when the adult step kids are around, and always have to defer, and not say what I feel as it will hurt him.