Mario Golf/WMG

Sherry and Grace are lovers
This theory isn't based on any hints between the two in dialog, but their vast mismach in skills. Sherry is fairly competent among the AI golfers, but Grace is completely inept, even among the AI golfers. The reason they stay together is obvious.

Alternately, Grace's parents pay Sherry a lot of money to keep her around
Mario wiki mentions she is spoiled going by her Japanese profile.