Unacceptable behavior from that family. As stated earlier, the grandparents should have visited the kids instead of vice-versa. I could have seen seen one visit up-cabin to oooh and aaah (would have disagreed with it but could have tolerated it) but 23 is beyond intolerable. I flew as a minor in E with my grandparents in F and my grandparents explained to me that I wasn't supposed to go forward. I was fine as I had books and very attentive FAs (this WAS 40 years ago) but it was pretty clear that visiting the front was just not allowed.