sounds like a combination of tiredness, and being aware that something is happening. Could the be a bit under the weather? my girls get like this when they are coming down ith something

I would persist with school as she may realise making a fuss means I get to stay home and might create a nasty cycle. Sounds like you are doing the right thing lots of love an reassurance. I feel keeping up regular routines can really help kids get through upheaval and change such as the addition off a new bub or a tired hugely pregnant mummy, this is especially true for our eldest dd

good luck, I hope your little ladies perk up soon xx