If your husband really likes Ethel then I think it is best to use it now as a middle name! Then it has been used which should make him happy (and if there is ever a dd4 it can't come up). Middle names very rarely come up. I've only been asked mine once or twice in the past five years and I only know one or two of my friends' middle names.
Daisy is fine, I personally am not very keen on most flower names as full names i.e. I love them as nicknames but not really as given names but at least Daisy is popular and quite classic.
So I would say go for if it works for your family. 