I feel for you op. My dsc are not treated like family with their step dad's family. I am not even sure they have ever even received a card from them. They are similar ages to your eldest so do understand, doesn't make it any less horrid for them though. And their steodad has been on scene for 6 years or more (was other man)
My parents see it as them having 6 grandkids (my dsis has 3, my dd and both dsc) and treat all the kids the same.
My fil on the other hand will give my dsc pocket money every time he sees them but never my dd, and she is his blodd family as she is dps (his son). It never bothered me much when she was a baby that he treated them differently but she is 5 now and she sees the difference. For example, dss mowed the grass this weekend and dd and dsd cleaned the bathroom for him and tidied his room, collected his washing etc. He gave dss and dsd £20 each and ignored dd completely. Now she is 5 so I have no expectation that she should get £20, but she would be elated at £2 but it simply doesn't occur to him. Mind saying that, he doesn't thank me for the hours a week I spend cleaning his house, doing his washing, taking him out, doing his shopping, managing his bills... The list goes on.
Your dp absolutely has to address this. there can be no two tier system in a blended household or it is asking for trouble. Everyone treated the same, whether that is gifts, pocket money, discipline, meal choices.
If he cannot see this then please postpone the wedding. You cannot commit to a life where your kids are seen as second class.