This is a good idea too, but I do see them as different implementations with different advantages.
“Following” is much simpler to implement, because it uses mostly existing systems. That’s a big bonus.
“Following” is essentially automatic cross-posting, right? Presumably, everything from the followed community is cross-posted to the follower communities. I can’t think of when I would ever prefer that over getting selective cross-posts. Sometimes I don’t want to blast stuff out to all communities. Sometimes I want to post something in a local community, and other times I want to hear from all related (sibling) communities. Maybe it’s just too centralized for me.
Siblings are related to each other but retain their unique identity. A followed person doesn’t need to know or care about the follower, and doesn’t have to allow any input from the follower. “Sibling” relations are bidirectional, while “follower” relations are unidirectional (though both sides can follow each other). I think all this has a big functional difference.
I suppose some of this is a matter of taste as well.