Autoimmune disease?

Your UC might be an autoimmune condition (no one really knows) but I think it's discussed that way in terms of your immune system overreacting to benign stimulation, kind of like you're having an allergic reaction to normal things in your bowel. (I'M NOT SAYING UC IS AN ALLERGY).

When you say your sister has "a flu," what do you mean? Does she have influenza (cough, fever, body aches) or does she have diarrhea? If she has diarrhea, you should probably stay away, because any bowel virus you catch is going to make your UC worse. If you catch influenza from her, you're going to be miserable enough just from the illness. The complications that UC might have for you is if you get a post-flu bacterial infection - treating with antibiotics can be complicated for someone with UC because antibiotics disrupt healthy bowel bacteria, which might trigger a UC flare for you.

Strep infections are not very contagious but if you get one you will have to work out the gut bacteria disruption from the antibiotics.

vomiting and nausea are not influenza. Sounds like a gastric virus.

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I think it's wise for IBDers to limit their exposure, as much as possible, to any type of infectious viral or bacterial illness. BUT, I'd say it's especially critical that they stay away from digestive bugs. The stomach or intestinal virus that's a self-limiting, 24 hour bug for the average person can land an IBDer in the hospital and cause weeks of misery. It seems hard-hearted, but I'd advise you to limit your contact with your sister as much as you can.

For what it's worth, I strongly believe my UC is autoimmune. Although I've learned how to manage my UC and have been in remission for years, other autoimmune conditions have popped up – Raynaud's, chronic fatigue – indicating I've still not resolved some underlying issues.