I'd be interested to know how many years these holidays have been going on for, and how old the grandchildren are now?
As we age, and as our grandchildren get older, I'm sure everyone has different ideas about how they spend their precious holiday time. Things change, and don't always stay the same.
Personally, even though it's upsetting to you that DiL has still arranged a holiday with her own parents, I'd just let things be, and try to accept that, for whatever reason, she doesn't want to spend holiday time with you. Don't dwell on the reasons why, and don't push for answers, as I think this could have the potential for causing more upset to you, or it could annoy your DiL and make your future relationship difficult ... it's really not worth it.
Holidays are important, and most folks want to spend time away doing what they enjoy doing, with people who they enjoy the company of, and not feeling obligated to spend time with others when they would prefer not to. Your DiL possibly doesn't feel as comfortable around you as she does with her own parents, and that's only natural, we're all different.
Concentrate on spending time with your other family members, who enjoy spending time with you.