https://dynasty-scans.com/chapters/kami_ona

Is that last page suggesting that the Mother is going to get in on it in the future?

It's law: if a girl in a Yuri manga has a sister and no defined girlfriend, the incest route will be taken.
Now, I don't know much about hair, clits or female masturbation and hygiene, but I don't think it's healthy to tie your hair to your clit. I could be wrong, though.
Oh, and third part will definitely be their mom joining the fun as punishment for wasting water.

It's law: if a girl in a Yuri manga has a sister and no defined girlfriend, the incest route will be taken.
Now, I don't know much about hair, clits or female masturbation and hygiene, but I don't think it's healthy to tie your hair to your clit. I could be wrong, though.
Oh, and third part will definitely be their mom joining the fun as punishment for wasting water.

If you tie your hair like that it'd likely be a bitch to untie. I don't recommend that. Plus cutting off circulation if you tie too tight would likely be painful. Buuuut some people are into that so shrug