Step kids mum and holiday

[gototheshop]

Step children both primary aged who we have 50% of the time. Holidays split in half.

We had them last half of summer last year and we’re set to have them first half this year. Have had a big holiday planned and booked since last summer.

Mum is pregnant and due during this holiday. We didn’t know this until recently.
She wants to swap halves so kids are there when baby is born and so she gets a break with newborn.

What is reasonable here?

Babies don't always arrive to schedule so you may change your plans at a big expense and then find out you didn't need to!
I was thinking this too unless she has a planned csection booked. Even then my middle boy still ended up coming a week before the date I was supposed to have my csection.