Why not give it a go and see how it works out. Children's needs change and she'll be able to go to nursery when she's three so if your daughter has a fair time of maternity leave you will only be needed for around 2 years (I did 6). And it is lots of fun. You'll build up a good relationship with your granddaughter that will hopefully last a life time.
As for your husband, he may never notice anything different except that his wife is happy and fulfilled - and has lots of lovely stories to tell if ever he wants to listen.
People have mentioned have mentioned how your other children might expect the same help. This is true but they ought to be sensitive enough to realise you are getting older and will not be able to give childcare for ever. Which is one good reason to do it now while you have the chance and feel you have the stamina.
My third grandson is due is only a few weeks but I've made it clear to my DS that I will not be able to do for him what I have done for his sister's children, much as i would love to - anno domini does eventually take it's toll.
Good luck
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