Adult sibling relationships

We have 6 adult children who are rad. (I can say this bc we adopted them and so I didn’t “make them.” LOL)

Their ages are 18-26. Four boys and two girls. Four of them live with us since COVID while they attend college. Two live independently.

I’m struggling with the relationships they maintain with one another. I think I had this manual where “families keep in touch” with one another once they are adults. (Stemming, I believe, from my own desire for adult siblings.)

As they age, I see 4 of them as very close. Their own text threads. Their own chat groups, etc. I tend to feel sorry for the two kids who are not included in those groups. As children, I would have discouraged them from excluding other siblings.

But now that they are adults, this doesn’t feel like my place. And yet it makes me sad.

Here’s the intentional model that I am trying to move to. I am not comfortable just swinging to this immediately. A bridge or ladder needed maybe?

C: We have 6 children who are ages 19-26
T: They were very close as young children and will continue to be close their whole lives.
F: Grateful
A: Invite them to family events that are at our home; create a group thread for everyone (parents, too); don’t expect life to be perfect; accept that there will be 50/50 to their relationships;
R: I feel good about their adult relationships and trust that nothing has gone wrong.

My problem is that I don’t believe this.