What's the chance of this happening?

Hopefully someone can help provide the calculations for the following probability problem...

Parents of a child at a senior school walk into the reception to pick up their daughter.

The receptionist is holding on the phone when they walk in and she says "Have you got the pupil number for Georgia Addison-Lawrence? Who is meant to be starting here in September"

The parents say "She's not coming here, because she was offered a place at a school closer to where we live"

The pupil the receptionist was talking about was also the daughter of the parents, the sister of the pupil being picked up early from school. What factors are required to calculate the probability of this happening?

I ask as I was the parent when this happened two days ago.

Thanks, Tony