Hooley dooley Ditto to what everyone else has said. I feel sorry for the poor DIL, no wonder she seems 'stressed' during visits. OP you really need to take a step back and get your own life, or you are just going to drive them away. Your son was happy for you to visit for weeks before his wife.... bc he had no wife. You would see claw marks down the walls if my MIL (or my mother for that matter) stayed for weeks. That would be my own private hell.
If you want to see your grandchild, stop seeing your DIL as stealing your son and some kind of weird competition for his affections, and start respecting their boundaries. It seems you are in a vicious cycle where they feel suffocated by you and are pushing you back, which is making you come on even stronger, which is making them take further steps back. Your son is middle aged, not a child. You still speak of him like he's 18. Please respect his wife and their relationship and you will find she'll be much easier to get along with.
And I should add that I think the reason why you've got quite direct responses OP is we are married women in our 20's/30's/40's with young children. Like your daughter in law.