I totally agree that the possibility of a job and the inevitable house move that would go with it are irrelevant, the law said one day, no stop overs over the xmas period, therefore you were all breaking the law!
That aside because it's been done now, most mums of young babies/children these days regularly have baby monitors to listen for them crying but if your daughter wanted to bring this into your home she should have asked permission and explained what it was for and where it would be put.
As far as the fact that she may have initially 'overheard' you complaining to your husband about her being idle and not helping whilst a guest/family member in your home goes, she should have made you aware of the fact that she could hear what you were saying because yes, it was a private conversation, but truthfully, if anyone 'accidentally overheard' a similar conversation, who wouldn't try to find out exactly what was being said about them? I would also say that rather than moaning to your husband, this is your daughter and you should have spoken to her about it and said that whilst she and her family are in your home there are expectations of help with cooking, washing up etc.
I disagree with the person who very early in this thread said that you clearly don't like your daughter because you were moaning about her. I know that 'sometimes' we might not 'like' things our loved ones say or do but that doesn't mean we don't still love them!
I think she's taken the hump about being expected to help but without being asked/told, she doesn't like the fact that you moaned to your husband, then her attitude after listening in to the conversation and being called out on it, is in my opinion, downright childish, she needs to wind her neck in and grow up, she's now a mum herself and there's no excuse for this pretty behaviour!