Could they be on holiday, or at work, or at the shops, or visiting Auntie So and So? Anything that you think might satisfy her for the moment, really.
I used to use 'at the shops' and 'visiting Auntie X' when my FIL started asking where MIL was. She'd been dead 10 years - the truth would upset him terribly and in any case he would only forget very quickly and ask again.
We mostly tell the truth (although she doesn't ask about people who used to live in our house but rather imagines her parents live next door). It doesn't seem to upset her as she is presented with the facts hundreds of times a day (every time she asks about them) and does have some 'trace memories' so isn't shocked. For her Mum/Dad I say 'No you're my Granny, and your Mum and Dad died before I was eeeven born but we look after you here and love you'.
When she is particularly agitated or upset I sometimes say 'they're not here just now it's just us, they might come later' but she doesn't believe it!
I think you just have to do what you can- sometimes it helps to make something up (which we do a lot for other things) and sometimes it's better to say what happened- you can decide what suits your situation better and it sounds like saying her husband is just on holiday is probably a good idea as introducing the concept that he died now might actually be much more difficult and make her very upset.