My sister-in-law has developed a habit both my wife and I find annoying: in order to try and get us to come to their house more often, she constantly pulls my son aside when they're leaving from a visit, and tells him to "tell mommy and daddy to bring you to our house so you can see x" (where x has been, variously, "our Christmas lights", "my new car", "our new puppy", etc.).
We both find this behavior manipulative, and it puts our son in the middle, since we then wind up having to tell him "no" more frequently than not.
As for why we say "no"... well, the full details aren't relevant, but their apartment is small, there's little for him to do aside from sit there and watch their TV (which is always on), and we want my mother-in-law to get out of the house as much as possible, since she's essentially a shut-in ever since she broke her arm, and was told she can't drive.
My sister-in-law drives her over to our house for visits frequently, and we also meet them at local restaurants for dinner regularly. In fact, we average seeing them at least once a week.
By contrast, we only see my own mother, who lives only 5 minutes further away by car, maybe once a month or so. My son actually gets particularly excited to see my mother because, according to what he's said repeatedly, he doesn't get to see her very often (he's explicitly compared the frequency to his frequency of visits to my MIL, e.g. "I get excited seeing Grandma and Grandpa, because I don't see them as often as I see Grandma and Aunt").
My son, for reference, is 5.
How, if at all, should we handle this? Should we say something to my SIL (who will, incidentally, burst into tears at virtually any criticism)? Should we say something to my son? Or should we just let it go?