My DH and I are like OP’s son and DIL. We moved away from where most of our relatives live because we enjoy a different lifestyle that couldn’t be had where our relatives live. Our relatives on both sides have vilified us for this. His relatives blame me for taking him away/keeping him from his family and forcing my likes on him. Mine feel the same about him. The truth is, neither of us was allowed to even express wanting anything different from our families of origin. They all live in the same area, socialize the same ways and vacation the same way. They’ve chosen partners who are happy to assimilate into this. We refer to the as the Borg (from Star Trek).
In the almost 3 decades we’ve been together, no one has ever visited us. Their attitudes are that we are wrong for wanting something different for our lives, so we have to come to them. We have made a handful of miserable visits out of a mistaken sense of obligation, that we regret wasting our time and money on.
We know they disapprove of us and feel uncomfortable and unwelcome. Our children can also feel it, and don’t like going either. They flaunt their “closeness” with inside knowledge and jokes that “we just wouldn’t understand because we chose to leave”. The homes that “have plenty of room to host us and are safe enough for them to have raised their kids and host their other grandchildren” are fraught with safety concerns like unsecured firearms, untrained animals and exterior doors leading from the children’s bedrooms that don’t lock. Because the local relatives aren’t bothered by these things, we shouldn’t be. It is even worse when we book our own accommodations. Then we are being stuck up, think we are too good for them, don’t know how to be family.
This is all massively dysfunctional. OP, I recommend that if you want a relationship with your son and his family, you need to make some changes. First, you need to stop gossiping and complaining about them to the local relatives, especially their siblings. You’ve created a hornet’s nest that your son and family couldn’t possibly feel welcome visiting. Second, you need to apologize to your son for refusing to visit and if the invitation is still open, book accommodation and visit them. Then, during the visit, be the loving mother and grandmother. Ask them to show you the seaside. Find joy in the area they love and have made a home. Don’t compare them to your other children/grandchildren, but enjoy them for who they are.
Mostly, don’t blame your DIL. It is most likely that either your son wanted to move to the seaside but knew this wouldn’t be accepted or that your DIL introduced him to it and he fell in love with it. Either way, he made the decision for himself to move. It is doubtful that she is forcing him to live there and stay away from his family of origin.