MIL has a young DD (just about to turn five) I completely forgot that mini-SIL was starting school yesterday 
realised after seeing MIL's fb status yesterday. So DP said to me last night that I should have invited his mum round yesterday. I thought he was being a little unreasonable as I thought she's done it all four times before. Anyway chatted to her briefly today and it turns out she went home and cried after dropping mini-SIL off at school. Now I feel guilty that I forgot and didn't invite her over here to distract her from her empty house.
We could have done with the company too as DS is recovering from chicken pox so we have a case of cabin fever here. (I get on really well with MIL btw, bar a few niggles over her helicopter style of parenting with her DD3, which I would never, ever mention to her!)