It doesn't have any *blatant* reasons - and it's certainly possible for Son and DIL to prioritize the big family without deliberately excluding the small family; after all the big family probably doesn't get all together very often, and they probably put a lot of work into it, whereas you can go see LW and Sister anytime, and it sounds like LW has been a single parent with no experience with large families and really feeling the difference, so it wouldn't be odd for that to cause complicated feelings they don't really know what to do with constructively.

But the "access to grandkids" thing combined with just the general vibes of the letter... yeah.