Both situations are possible. I am losing one this summer to school but he is going to the school based daycare so his sister moving too isn't an issue.

I am not in their school zone anyways. In that case it is also very common for the mom to move both kids to a daycare in their school zone when the older one is ready to start. Also a reason I don't like taking kids back after mat leave if there is only one more year till the older one starts school.

Depending on the options for school age kids in your area will determine what happens. I would get in the habit of thinking of your daycare as a 2 and under daycare and start assuming that when mom goes on mat leave that you will lose the older child too since by the time they would be ready to come back it would make more sense for mom to take the year of mat leave and find someone in her own area that can take both of them.

Also even if the mom says oh we will leave the little one with you and just move the older one. If down the road an opening comes up in the other daycare the mom may be put under pressure by the new caregiver to transfer the sibling over too under the threat of the caregiver going to fill with a baby/sibling team. Had that happen to a friend and a month into the school year the mom moved the sibling by which point no one on the caregivers list was still looking for care and well let's just say she was not a happy caregiver. One of the reasons I am so happy the way it worked out for my sibling pair and I will have her for two more years as she is only 18 months now and she will be first on the list for the school care because her older sibling goes there so mom is very happy too. And I open at 6:30 for her so that limits my competition.