Should I stop sister visiting elderly mother?

I think my sister is living in a parallel universe, where there is no Coronavirus. She is a primary teacher, so mixing with children every day, and is planning to visit our elderly mother in her care home next week. The care home has not (yet) introduced a blanket ban on visitors, so it is left up to family members. I’m more or less self-isolating and I’m not going to visit, just in case. I’m phoning often instead, and in choosing this course of action I’m thinking of our mother, but also all the other residents. Should I try to dissuade sister from going too?