If your Mom was dying of cancer, but telling her she would get better would extend her life and give her strength, would you lie to her?
If you mean I was aware of the cancer, but she wasn't? This implies I have a time machine and I have proof she'd live longer by lying? Well, if I had the time machine, I suppose I would be tempted to enjoy having her around longer, by indulging in a lie of omission about the cancer she has...
but then I'd be playing the role of God, wouldn't I?