I think her role would just be too insignificant to justify her appearance. As far as I know, her contributions to the plot were just that she went to this spa with her sister and that she has a daughter. But who knows, maybe we'll see her in the outro. And let's face it, in a plot with such a number of colorful personalities, I don't think it's an excessive loss either.
As for the interaction between the mother and her sister, just because we didn't see them interacting with each other doesn't mean they were talking on the phone or anything. For example, I could imagine that while the protagonist was in the village with the girls, one of them called the other, along the lines of "Hey sis, our kids are out for the weekend, let's go out and get drunk."
On another note, where do you think the protagonist will go with all the ladies (and presumably Mike as pilot)? For me personally, the only place that makes sense is Japan, since depending on how the MC scored, they have a yakuza there that can definitely be considered a family friend.