Have you ever wondered what the other champion siblings that aren't in the LoL are up to? For example, Katarina and Cassiopeia have been stated to have at least one other sister, Fiora has numerous older brothers, Lee Sin has a sister, and Irelia has a brother.
What are they doing while all of this is happening?

Not everyone in the family is League material.

True, but it's still curious as to what they're doing. I mean, is Katarina and Cassiopeia's other sister as focused on finding their father as much as they are? What do Fiora's brothers do now that she's the head of the family?

I doubt we'll ever know since they aren't that important so far. If they do something that is front page news, get the Leagues attention, or become Champions we'll hear from them. Unlikely though since we don't need entire family trees clogging up the League.