Not really surprising,that's why he still doesn't want to end DC and keeps adding useless new characters and mysteries

Well, I think he doesn't care about money anymore. XD He could stop his manga today, he would still have enough money to live the next 40 years. All the characters who have been added recently are on the editors' demand, who want the manga to continue, because it's prepublication magazine, Shonen Sunday, is selling less and less magazines every month.
So, I guess theory about Yukiko and Sera's mum being sister goes out of the window. Hmm, maybe one of the sisters is Sera's mum actually, since Gosho only said that Yukiko isn't a sister, well I'm not sure.
Sticking with the "Sera's Mum is Bourbon's mum" theory personally.

But if Gosho says "no" for Yukiko, and ends his sentence by "but...", I think it means that of the two persons (Yukiko and Sera-mum), one of them is the sister, Sera's mum.