Blended family seeking to treat all children fairly.fair or unfair plz help!

My stepson moved in and my husband's parents are rich, the stepson will be going everywhere and getting everything with them while other children get nothing or go nowhere with them. Some of the things will be done to be spiteful. Husband doesn't want to denie his son. He asks if it was mine what would I do and he wouldn't want to stop them if it was the other way...personally if they can't all go then none go is my philosophy. I say sometimes and special occasions for this child to be treated special ok, but not all the time or when it's being done spitefully....HELP PLZ!! Thanks in advance!