I would find it upsetting, particularly if it's something you've been doing every year. However, it could be that DiL would like to break with the habit of going away every year with both sets of grandparents. Maybe she just wants to start having holidays with her own husband and children, so she'd prefer just one holiday a year with GPs ... maybe it'll be your turn next year ...
I can also see that the fact you were taken very ill last year, whilst on holiday with them, may be a reason why she's not so keen on going away with you again. Myself, having had a parent with medical issues, we had a few holidays cut short, and even cancelled. I got to the point where I didn't really look forward to holidays with them, even though I loved them very much.
I think you need to try to accept that this is the way it's going to be from now on. It's up to your DS and DiL to sort it out between themselves. Don't get in involved in questioning why this has happened as it will only cause more problems. Sadly, it's usually the wife who gets her own way in these circumstances, so you need to accept that, and try to do other things with with your grandchildren, such as days out, like others have suggested. And enjoy spending time with your other family who like spending time with you.