@Xical I think you have the wrong impression about onee san relationship.
1. If its about the one who exchanged sake, their relationship isnt about working in the same dinner party or attend the same patron but more like Yakuza sakazuki ritual (if you already read yakuza themed series you know how this work, its basically the same). So if they attend the same party, at first its to advertising their little sister to her patron (and guide her if needed), the next can be a full coincidence (like some chapter ago).
2. Its the same too with others onee san (your senior), they basically just your work colleagues (just more senior and because there is a tight hierarcy system at work, you must respect them).
In short, when you attend the party, you already working full time. About the price, Maiko definitely cheaper to meet rather than a Geiko, even though I think to meet a rising star like Momohana can be more expensive than a normal Maiko.