I think it depends, how long has the overseas holiday been booked for, was it before you were PG because if it was then i don't think it's all that fair to be angry at her.
DP's mum booked a cruise right as i found out i was PG, we called her and told her that i would be due the week before she was meant to leave and gave her the option of canceling... she didn't as we never expected me to go over and if i did she agreed it would be her loss but she was only going for a week.

Is there a particular reason you wont change to a sunday?

I can completely see where your coming from and it's up to you when you have it but personally i would be telling her thats when your planning on having it and see what she says, explain WHY you want it then and that you don't want it too close to your due date but i think it's only fair to mention it to her... even if it is to say i've set a date for the baby shower, are you still in Aus then???