@Thisisit2323 I don’t think this is worth destroying your nuclear family over.
You will have more power when you are married and not just a gf anymore and I hope you are able to move past this to get that legal security for you and your kids.
I wouldn’t let it bother you and I would be united as a family of 8 on all family communications. Push it in their face that you are a family and all are equal.
I would have DH, post wedding, bring it up to them that the unequal treatment of the children, if still going on, is causing distress with all the children and try and solve it that way.
To try and let you see it from their point of view - personally if my son took on another 2 kids on top of his own I’d find it hard - because time/resources for GC would be less all round - but I would get them something at hols/bdays and not leave them out. However, I wouldn’t be paying for them to have days out and all that, nor expecting to have much of a relationship with any of the kids that weren’t my actual grandchildren beyond about 18.
For example, I wouldn’t contribute to their uni costs, house deposit, wedding or anything like that or be very involved with them emotionally because that would deplete my financial and emotional resources for my grandchildren who are my responsibility… it literally would not be my place to be taking on grandparenting children I am not related to.
To be fair OP, your youngest is 11, the time for gifts is about to peter out in about half a decade, and GP won’t be around forever either…. I wouldn’t upset the apple cart for the sake of your long term future.