D.P.
The number one important thing with grandparents and grandchildren is safety. If the grandparents don't watch babies carefully, or let them play with a jar of nuts for example, and you don't think that's safe then you have to say something and say it forcefully. Make your stand and choose your battles wisely, but safety is non-negotiable. Beyond any safety issues, however, its better to back off. Just let your in-laws (and parents) enjoy their grandkids. Don't make a big issue out of nap times and schedules and how they relate to their young grandchildren. Just let that relationship take its own course and try not to manage it too much.
The bond between grandparents and their children can be a very important relationship in a young child's life. I clashed with my in-laws early on when my kids were young, but I realized it just wasn't worth fighting with them about how I wanted things to be done. When other people interact with your kids, they learn about being flexible and having things done a different way from mommy and daddy. And you have to figure, these people raised a kid you you ended up falling in love with and marrying, so they can't be that bad.
Just relax and try not to take things too personally, even if they are meant to be so. Focus on your beautiful twins and try to enjoy this time. Good luck!