It makes it sound more like the sister doesn't want to hang out with the mom that night and is pushing it on to you instead, so that would make a lot more sense.
Sounds that way, yeah.
Now I'm wondering how your suppose to trigger the event since they both have to have a reason to do so. What would cause them both to seek comfort in each other? I know the mom's lonely but the sister just wants to hid her secrets but the mom already knows them. So what is the thing that sets the sister off then?
As far as my interpretation goes, the MomxSis path is supposed to parallel the MCxMom path. Key difference being that the expectations are different.
On the Mom’s side, she has some kind of empty-nest syndrome
and is suddenly abused by / feels abandoned by the "father".
Both Mom and Sis are also affected by the sex world influence to some degree, presumably.
The MC is going to school, has vague plans for university and the Mom is afraid he will leave the nest, triggering her feelings of abandonment. As a guy, the mother also wishes for him to "man up"/ take up part of the role the father seems to have abandoned. As a consequence, their relationship is very emotionally charged on her side. She takes out her frustrations on him, then feels guilty etc.
The sister has dropped out of school and has a job (two, technically) in the same city. She is less of a trigger for the empty nest syndrome. Additionally, their girl-to-girl relationship more easily lends itself to casual intimacy. As such, their relationship is much more mellow and positive.
Of course, several parts of the game feel mashed together with very little regard for coherency. Who knows what Geeseki or the writer actually intended?
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Unrelated, but the new update seems to be done. Jan 24th was the last time Geeseki marked anything complete and just now, the remaining to-do list was merged into the to-do list for the next version. 2-week early access should release tomorrow. Full release presumably in 15 days.