Heh, that ending. 'I wonder how much mom knows'? Well...Surely, she knows everything!
Or rather, she probably arranged for everything.
Next thing you know she's going to start asking to see her grandchildren.
Kubo-kun: "I really wasn't expecting any sort of Mother Inquisition."
Hostess (accompanied by jarring musical chord): "No-One Expects the Mother Inquisition!"
Seriously though, A Mother in Hentai KNOWS All, and, even if she didn't say so explicitly, her tone and dialogue on page 18 essentially told Kubo-kun: "You'll definitely Take Responsibility regarding Tsubaki-chan, Right?"
And we who love Vanilla themes would likely not want it any other way.
"Really now, how do you think I met your father?"