Who are the members of the Morbucks Family, what what do they each do? does princess have any cousins?
From the number of Morbucks NPCs shown in the game, it seems to be a pretty big family. The ones I've found in-game work the banks (though I swear there were more, it might be my memories of Retro crossing over), so I'll list them as well as my headcanons for them below:
JaeHoon Morbucks: JaeHoon can be found running the bank in Peach Creek Commons and wears an Urban Ranger uniform. As a ranger, he's learned a great deal more about responsibility, charity, and teamwork than others in his family. Some other young Morbucks' think he's weird for joining them and wonder why he would ever want to stick around a small place like Peach Creek, but he genuinely enjoys roughing it more than they do, as it feels like something he's personally succeeding in, something outside of whatever the family throws him into with their limitless support. He is a more distant relative, but despite this, is still pretty well-known in the family for being very bright and already having a good mind for business.
He and Ranger Joey are really close, in that they're both scouts as well as in that they'll sometimes match each other's styles. Jaehoon respects that Joey knows how to work a deal and Joey admires how far Jaehoon has gone as a young tycoon in a giant family business. Chances are, this friendship will last and they may try some real business endeavors together in the future.
Jerry Morbucks: Jerry is located at the Townsville Mall, the same as Princess. While he's been tasked with helping keep an eye on her, he'd much rather focus on his own interests and can be somewhat lazy. He basically does as much of his work as is passable and then runs off to have fun when he can. We can take this both from his position in front of an ice cream shop at the mall and in the way he dresses (in the DexStar outfit), which further hints to a love of superheroes. He's also a big comic fan. Jerry and Princess can bicker quite a bit even though they're both on the lazier side, threatening to tattle on each other to their relatives. This stand-off can mean that a lot of the trouble they get into separately goes unreported for the sake of not having their own dirty laundry aired out.
Prince King Morbucks: Prince is located at Mt. Blackhead and is the most like Princess in looks and personality—so much that he's basically her 'Rowdyruff' counterpart. Like her, he has no concept of frugal spending, can be arrogant, and is used to getting what he wants. However, unlike her, while people can find Prince obnoxious and self-absorbed, he's not actually that bad of a person. He's spoiled, not evil. He'll throw money at anyone to have his way and does get upset if he still can't get it, but will react by fuming alone to himself instead of having a tantrum or hurting people. He wants to be in the groups that hold his interest, such as the Candy Pirates, but instead of getting vengeful from being excluded, he just gets really hurt. Instead of Princess' cycle of abusing the money she has to lord over others, while also knowing that the main reason many of his "friendships" exist is because of his wealth, he still feels a need to use it to impress others and be accepted. He's also very active, in contrast to Princess' stronger desire for comfort and reliance on tech.
I imagine he likely has the closest relation out of the Morbuckses I've mentioned. One common reason behind some middle names is to pass on the name of a relative. Given that his middle name is the same as Princess' father's first name, King, one of his own parents could possibly be the latter's sibling or close cousin—making he and Princess very closely related in-turn. Due to their differences, the two don't always get along.
There's supposedly at least a fourth Morbucks located in the Foresaken Valley, however, aside from records on Retro such as the patch notes, I can't find them: I know they were in Retro itself, but I've checked both versions on OpenFusion for the original game with no luck. (For some reason, I'm having trouble loading Retrobution as well. :/) This also doesn't include the Retro exclusive Morbuckses from when they added all of the additional bank space.
One thing that was also brought up in the Retro patch notes and that I noticed too was that there originally wasn't many female Morbucks NPCs. For the sake of headcanons, we could fit this in with Princess being a spoiled daddy's girl in that there might not be many other girls close to her age in the family, therefore giving her some special treatment within the Morbuckses as a whole.