Yes, he has. And that's unfortunate.
But this is his child. His child that he hasn't SEEN for nearly a year.
You cannot expect him to pass that up in order to not let your children down. I'm sure he thinks the world of them and he is going to come down for a couple of days to be there, which is good.
But if I hadn't seen my child for a year, and then I was told I would actually be able to have them for a whole week, well! no promise to anyone else would mean a damn against seeing my precious child.
What do you expect him to do? Turn down the week with the child he hasn't seen for a year so that he can spend 3 weeks with your daughter?
I know your child means as much to you as his does to him, so of course you feel this way, of course you do. Apart from anything else, your child is going to have an operation and you are bound to be feeling awful about it.
But turn it around.
Your own child. You've not seen her for a year. You were going to spend time with someone else's child but you get that phone call - you can see your child.
What do you do?
Say no? Say well, that's great but can we possibly rearrange, only I'm going away for a few weeks?
Hell no you wouldn't!