so they have a 4 month old, a 3 year old and both work? sounds like what it was like 'back in the day' with us.
when ds was 4 months old dd was just shy of turning 3, and i had just gotten back to work after maternity leave. while dd loved her new baby brother, at 4 months of age he was not that cute little bundle that (as she put it at the time) 'just sleeps, eats, and poops'

. he was, quite normaly-at a more demanding stage and as such dd began reacting, plus both were in daycare again so that was another adjustment for them (vs getting to be home with mom f/t). most weekends dh and i were trying to play catch up with the household chores, trying to eek out some special time for dd and generaly keep from collapsing from exhaustion.
for our own sanity we had to turn down many weekend (and esp. evening) visits. and i'll be honest-the grandparents (and uncles) were invited over far less often once ds was born vs. when dd was born because it seemed like company just set dd bouncing and got her off her schedual such that what family perceived as 'just a short visit, we won't disturb your schedual' left dh and i with several hours post visit of trying to wind dd down, trying to get the baby settled...it just seemed better when both kids were a bit older.
how does the sil know the other grandparents are there 'all the time'? is someone telling her this or is she just assuming it? it could be that they are doing what i've had experience with a couple of family members doing (that dh and i put an end to not without some strong words and actions)-not taking a polite 'no it's not convenient for you to come over_________" for an answer and just showing up at the door


some adult children can't bring themselves to turn relatives away that do this so in some cases it can account for one side of the family seeing grandkids more than the other.