WooHoo - I've been rumbled!! - I am an unpleasant, interfering old MIL and my DILs hate me, and I go around messing with rockery and leaving flowers for my DILs and all sorts of other awful things - I've looked for cotton wool in their bathrooms and other things too gross to mention 
OK - I'm pleased you're laughing (honest) you want to know why I think asking MIL to come and wash windows once equates to them coming to my house (is the house just yours, or does DH own half of it) to wash windows whenever she wants. Now call me pedantic but when you first posted about this you didn't say you had asked her once before, but be that as it may, I think your request could well have given her the idea that it was something she could do to help a working mother of 2 children and pregnant with a 3rd. No? I have a Q for you - do you like washing windows??
I'm glad MIL is a good grandmother but I don't understand why you turned her down to care for the children. When I started reading your para on this I thought you were concerned for her health and didn't think she was really up to it, but it seems your concern was around her reliability (or lack of?) I may be wrong of course. She sounds very energetic for someone in poor health - gardening and washing windows!
Ah Firesidechat you come late to the party. I was waiting for someone to tell me that I was not "helping the bad names that MILs get" - that comment usually comes much earlier on! And is there a reason why I can't use bold to accentuate a point (as everyone does on MN) is it because I'm a MIL
Brock I enjoyed your post - you sound very level headed but I have so often been accused of "defending MILs everywhere" happens every time. Of course I'm not - I may be old but I'm not stupid (well not very, anyway) Can I turn it round and say "why are you DILs defending DILs everywhere" - no difference really. I'm always struck when I get this comment about "defending MILs everywhere" why this never occurs to DILs. Strange really.
Tequila In your original post about MIL's bizarre behaviour you said it was after her marriage imploded "that's when she went a bit weird" - I made a minor mistake and thought DH had died rather than the marriage had ended, but both would be a bereavement in the sense that it is a loss. People tend to think only death is a bereavement but it's not the case. So No I don't owe diddl an apology - I found her excited request for a thread/thread/thread in very bad taste. When I posted about this you said MIL was/is batshit so why did you say originally that it was after the marriage broke down that she "went a bit weird."
Oh and apologies again saying "mom" instead of "MIL" but your comment about sherry is ageist. Not all older people drink sherry - I drink Stella and red wine
Mintyy "not helpful or clever of you" (a comment a parent might make to a child) I am certainly not "trying to get the OP to "bend to my pov and shouting more loudly (no I'm not shouting) I'm simply putting forward a different view from everyone else. And please tell me where I have called the OP a mil-hater. Don't make allegations that you can't substantiate.
and finally Iggystrop - it's bad form on MN for a poster to try to draw other posters into your own point of view, not that you'd have any problem with getting agreement here - in spades I reckon Are you a mind reader as you seem to think you what my perception of myself is - whereas in reality you have absolutely no idea. And yes I'm dead scary - as I said in the beginning you've all rumbled me.........Ha!! But ONE MIL has posted that the OP is not BU - priceless, so that must mean she isn't because a MIL has said so. If you can't see how ludicrous that comment is I am not going to explain.