I think you'll find it's just "A Wife and Mother". But I agree, this update doesn't advance Sophia's story in the slightest. It was even promoted as a bonus without Sophia.
Which is pretty stupid considering how long updates are taking. The 20th will mark two entire years since v.150 was released. So a fortnight from now will be approximately when L&P started working on PD13. And this PD has three events just for the bonus storyline alone (Ellie discovering Logan is cheating on her, the diner scene, and now this slumber party).
And yeah, Ellie hosting the slumber party would have been great, particularly on her route when she knows that Sophia is spying on her, like we find out during the Ellie/Julia event. And we find out that Ellie made head cheerleader. The only problem is that on the Ellie route, she's made every effort to make sure that Dylan isn't around - for example, she specifically asks Sophia if he'll be at home before inviting Jules and Elena over on PD8. And she does it when Dylan is at his football match.
So Ellie hosting the slumber party would only happen if she could guarantee that Dylan wasn't going to be there.