Leaving kids behind

I know a lot are saying that by the time they are 23&18 they will not need parents, but a 13 year is unable to grasp that concept yet
Exactly 62Granny.

And the older girl, knowing that the younger will then be the age she is now, may be concerned she'll be expected to take some sort of responsibility for her.

Red I think you should mention to your daughter & husband that the girls seem, to you, to feel unsettled by the talk of leaving.
You could suggest they discuss their ideas when the girls are not there and/or talk about what the girls' living arrangements could be too.

My parents moved abroad with my father's job when I was 17. I chose to stay in the UK. I stayed with my Grandmother till I married a few years later. Not a problem. These daughters may take the same view when the time comes.

This happened to two young sisters I know. They were told by Mum that as soon as they were 18 she didn't want them in the house. This was just before the eldest was taking her uni finals and she had a melt down unsurprisingly. All family offered her a home if necessary at any time and we take care to involve her in family news etc.

She did respond by getting a first class degree with honours in science, won a prestigious prize, and 2 months later was off to Indonesia to organise teams to bring business to outlying communities. Then she is off to Iceland to start her masters. She has rallied magnificently.

So on a sample of one, I would suggest providing support, and a listening ear, somewhere to go if needs must. The best revenge is then for those abandoned to do really well, as best as they are able.

You need to talk to your daughter.

As much as I know that they're very much independent by the age they're planning on moving abroad, at the age they are NOW it won't feel like it to them and THAT'S the issue.