When M-Gran shows up, you meet her in the parking lot. From there, you head to the girls room. Who's luggage is in the room depends on your roommate at the time.
You have to have triggered good old fashion family time with the sisters and run solo on the Aunt (family fun time with Aunt and Mom also counts). Once you have those, you hang out in the parking lot on nights when Aunt is in the alternate room and Older sis is at work. When OS returns from work, there's a chance Aunt will be peeping on you. If you have that kind of RNG luck, then you, OS and Aunt get together for indoor sports. THEN, once you've accomplished those feats and recovered from the massive outlay of energy, spark up a group fest with the sisters in one of the rooms on a day the Aunt is at the motel. She'll come barging same way the mom does when you unlocked that fun time.
Still not developed enough to give a definitive answer.
