A lot is made of the Middleton’s being invited to things as if it’s new, but the late Queen was obviously very aware that people who marry in have their own families that they care for for a very long time
That wasn't always the case, though. It used to be that the ILs of royals were pretty much excluded from royal events. I think that started changing when William was dating Kate and got so close to the Middletons that he wanted them included.