A whole weekend off once a month - well I certainly neither got nor expected any such thing when my children were growing up. You have your children; you get on and take the responsibility of looking after them and bringing them up.
This is quite crazy. If they were asking you to do it as a one-off (a special anniversary perhaps) then that would be different. Tell her you will do it if she pays for a mother's help to assist you!
I can understand they might ask you (although it does seem a bit of a cheek) but to start cutting up rough when you say you do not feel able to take it on is entirely out of order.
What a very difficult situation for you - I do not know what the answer is, as I realise you do not want to fall out completely with them - however, I would not fret too much about that as I suspect they need you more than you need them!
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