Because generally if one kid has the stomach flu or other contagious virus the other one is going to get it too but it is just in the incubation period ... As well the sibling would be carying the germs of the infected child on their person and would then be bringing it into the daycare and touching the toys/books/furniture ect and transferring the illness that way. I would rather try to keep as much sickness out of the daycare as possible. Same as if one sibling throws up or spikes a fever While at daycare both kids need to be picked up as the other child wont understand why xxxx gets to go home and they don't.