Because I know someone will ask, all of their grandchildren are attending that kid's party. This is an incredibly important plot point that everyone absolutely needs to understand, and certainly not just a joke for that last panel.*
* This is most definitely a lie
So yes, I AM pure evil for ending Part 5 where I did, but Part 6 is titled Maybe Sisters, and will have Nanase and Ellen finally talking to Susan about Diane, so that's gotta be worth something on ye olde alignment meter.