TI agree with rcat. Saying that she is ill and needs quiet so isn't taking visitors would be appropriate. If someone continues to ask (which at that point is rude) you could say that it is a private family matter and if she wants to share it when she is able to talk to them she will. If they continue to harass you just walk away.

I would suggest not coming up with a lie. That would require her remembering it. If you say she went on a trip or something people are going to want to hear stories. People don't want to hear stories about being sick.