The author of mar.anomy.net (an Icelandic blog!) suggested that I also list “siblings” that are connected via children as opposed to parents:

One thing that I’ve noticed is that many blogs don’t have any parents listed
and thus there is no way to find similar/sibling blogs to those.

I was wondering whether you had considered the option to list other blogs
that inspired the children of the blog in question.

My reasoning goes as follows:

If blog “A” has 50 children and 25 of those list blog “B” as their parent
also, then it could be pretty safely assumed that blogs A and B have
something in common.

That’s actually not a bad idea and the relationship is sort of equivalent to the connectiveness via parents. I don’t want to use the term “sibbling” though. Maybe “spouse” would be a better term, since these blogs are parents to the same blogs as the active blog.

Of course, some of these “spouse” blogs could also be “sibling” blogs - but I suspect that there is quite a bit of incest in blog relationships, anyway.