Except if she was the "is she the sister?" the MC mostly ignored her. He didn't try to talk to her in any real depth about her life, her upbringing, siblings, etc. She's been sidelined while the MC (and the game) have focused on seeing how many chicks can fit on his dick. Quite a few characters have received more time than Stacy - Jasmine, Natalie, Harper. Stacy isn't included in any group activities, not at the party and not at the new house tour. I don't expect to see her at the charity bowling event because she isn't an athlete, a girlfriend, or a cheerleader.It's actually the opposite. Stacy having the "is she the sister?" question over her head meant that the plot could not delve too far into her situation, otherwise it would spoil the red herring too quickly. Now that the sister question is resolved, we can actually start learning more about her situation, starting with this "I've been dealing with some things. I'll explain later" line. Her story is now exclusively *her* story. It also means the MC is now free to pursue her as the player wishes.
Don't take this as me hating on the game, I love the game except for it's severe lack of redheads. I was excited when Stacy was introduced way back when but she hasn't really done anything since.