Yep, got the in-law problem here, too. I sit and CRINGE while not only has MIL woken DS from his nap but then she walks him around her house and shows him EVERYTHING: "this is your cousin's Sponge Bob doll....and this is a painting of a horse, and this was your grandad and I on holiday, and this is his yellow and black striped shirt, and here is the TV, look what's on!" and on and on and on.
What I often have to do when we get home is take DS into the bathroom - the only room in the house with no windows and so can totally black it out - and calm him down COMPLETELY, because by the time we get home he's screeching from both OT and OS. At his next feed and A time I have to be suuuuuper chilled out and keep him relaxed and not do much and do a shortened A time with a long quiet and cozy wind-down.
And then pester DH to once again speak to his mother....