I would tell her, and use this experience to open up a discussion on them coming up with a back up plan for when your sick (maybe have the discussion in a week when your better though). I know the guilt, I live in PA and last Friday when we got 6+ inches of snow I wasn't able to make it into work for the first time in over a year of working for them. I have to be at work at 630 before the plows have really done enough to make it safe. I explained that I wouldn't be able to make it in till later that morning unless they wanted to pick me up or drop off my charge. As nice as the family is, DB still made me feel guilty about it. However, it sounds like your NF takes it to another level and you may as well try to discuss it with them. And as for telling her about the flu, you should definitely tell her, you didn't do anything wrong by getting sick and it totally justifies you taking a sick day.