Wait, isn't it the same author from that oneshot in which protagonist's mother finds her collection of mother/daughter books?
An age gap story this cute? On the first page? Day already made, and it's 1.45 at night. Probably the best one shot in quite a lot of time. Love the art style as well.
Also, the plot of this one reminds me a bit of this picture http://dynasty-scans.com/images/4306 It's not a cookie-cutter copy by any means, but one of the closest you can get plotwise.

I need to know which manga you are talking about, i've never seen a manga with mother-daughter incest with the plot you mention.