A former colleague of mine did the same thing. She has three daughters, all at the same university but in separate housing. The freshman caught it first (who knows, really) and gave it to her sisters. Then they all came home with mom and dad's blessing, and gave it to their younger sister and dad. The four girls had various mild symptoms, ranging from the freshman, who had "flu-like" symptoms, to the high schooler who had "allergy-like" symptoms. Dad ended up unable to get out of bed, unable to breathe, and had to make a trip to the emergency room (though he was never hospitalized).
They're all fine now and back and work and school, but it could have been so much worse! My friend says, "when you're sick, you want your mom". That's all well and good, but damn. Those girls could be suffering horrible regret right now, all because they needed mom's chicken noodle. I could see if they were dangerously ill, or if the freshman was all alone there. But she had her two big sisters, who were already exposed/positive, to watch over her, and her parents are only a couple of hours away.
My ex-co-worker is an otherwise intelligent person, so I'm giving her the benefit of the doubt and telling myself that maybe by the time the girls had symptoms and were tested, they had already been home for a visit and they figured the damage was done.