Hey, friends.
TLDR
I wrote a behavior to allow families to pick sibling objects, including other family siblings.
https://www.construct.net/en/make-games/addons/365/sibling
About
I saw that someone on the forums created a similar addon, but couldn't find a link to it anywhere.
So for my own game, I developed an addon to allow families to pick other sibling objects, including families. The original implementation was better but used undocumented SOL APIs, so I changed it to just enable you to grab the sibling UID, then pick it yourself. It's a bit less intuitive, but still works really well and am using it in my game.
I've many composite objects, using containers. And now the containers can benefit from abstraction.
I didn't originally plan on releasing it, but I figured it would come in handy to others.
Let me know if you find it useful :)