don't you find that sometimes people offer to do favours for other people because actually it makes them feel good to offer? for a split second, until they realise that actually, it is a big drain, a big obligation, that maybe it might even cost them? And then they um and ah about actually withdrawing their offer because they don't want to be rude, but eventually have no other choice and so do so maybe at the last minute, too late for something else to be arranged?
I have learnt this lesson - both as someone who has offered to help out a lot in the past and as someone who has been badly let down at the last minute by people i was relying on to do me a favour so it is actually kindest to be assertive and direct right from the start.
learn a few stock phrases to trot out if someone catches you unawares - that won't work out for me, i am very busy i am afraid, ect, and teach your husband too. But saying vague things like 'i don't know at the moment', 'i'll check and get back to you,' 'i'll have to see how things go,' - these aren't the same as saying 'no'!
Also, of course, it was very easy for your hubby to agree to help when it was YOU he was volunteering. And now he won't even answer the phone to her! If she phones, pass the phone to him. If she approaches you in the street, tell her she will need to talk to him. This is, i am afraid, a problem of his making and now he has to deal with it!
Good luck, you sound very kind...... x