Nyah-sama said: why is this in "the yakumo family"? they make a very slighty cameo here, seriously
Because it shows them having a moment as family within the pic. The pool is not limited to them being the only three in the pic, that is to say.. if they are participating in event as a family, then it qualifies. With that said, however, it's not likely that you're going to see many pics like this one in this pool because many show them together, but not out-and-about with a clear family-like tone.