I can sooooo relate to your situation right now!!!!! my PIL are always having to put SILs kids before mine because SIL is useless!!! shel just sit on her butt while her children are in the bathroom smearing toothpaste and handcream everywhere!!!
PIL had bought a highchair, sippycups nappies etc for their house when the grandkids come over and the highchair and cups were aimed at my boy because he is only 21 months whereas her youngest is almost 3!!!...YET SIL has taken the highchair back to her house and all of the sippycups!! so i bought 1 and clearly stated that it was there for Astin...well we have been there for 2 faimly BBQs recently and her child has the cup while we are left to struggle with Astin having an adultcup!!!

sounds silly i guess but it all starts to grate!!!
Sorry about the ME post!
I would suggest some PIL time without the rest of the family. We have been inviting PIL over every sunday so they can see my boys by themselves!