I had a lot of problems early on with my PIL imposing themselves on us just because we had a baby. They were perfectly happy to see us 2-3 times a year before that but once grandchildren arrived they seemed to want to come and stay every 3 weeks or so. All driven by MIL, my FIL just does what he's told.
My MIL raised a son who does not like to communicate with her (mainly because she is a pushy, passive aggressive pain in the arse). My main boundary has been to make this all her problem. She defaults to talking/emailing/texting me because I'm polite so I respond, whereas he ignores her. I have had to train myself to fight the people pleasing instincts. We go in a cycle where I spend weeks and months diligently forwarding her emails to him, not answering the phone when she calls and he's not in, and replying to some of her texts along the lines of "DH is sorting this out, you'll need to ask him". She finally gets used to dealing with him, and then eventually I always trip up and accidentally answer one of her questions and the floodgates open. She starts contacting me multiple times every day and I have to go back to completely ignoring her for my own sanity.
As a PP said, I have my own family to deal with, I'm not taking on the wife work of dealing with someone else's too. Forcing all communication to go through her son has worked well as it means I don't have to lie to her and if she's raised her son not to prioritise his relationship with own parents she really can't blame anyone else for the outcome.