Apologies Emma, I took

can't she see that when her daughter has kids (please hurry the F&%k up!!!) thats when it will be her turn to be the 'more involved' grandma?
to mean that you thought maternal grandmothers deserved more time with the grandchildren.

But as I said in my initial post - if you don't want her to babysit, you should feel able to say no. If she's rude enough to demand to know why, in front of friends... tell her that she's not responsible enough because you have SEEN her not take care of children she's looking after before. More than enough of a reason for me.