I have a dilemma and I don't know how best to solve it. Advice would be greatly appreciated.

I have OCD, which centres around fear of contamination and illness.

My sister was ill yesterday, but I am not sure with what. She has now informed me that her and her family are coming over to my house tomorrow. I am getting lots of intrusive thoughts and my anxiety is through the roof worrying about it. While my treatment is going well, with real improvements showing, my therapist and I have not yet reached the stage of treating triggers that produce this level of anxiety.

My sister knows I have OCD, but she has never once asked me about it, and we have never actually spoken about it, so I can safely assume she has no real understanding of how bad it is, or what 'sets me off'. (In my experience most people treat OCD as if it was almost a joke).

What should I do? Lie and tell her that I am away for the weekend so don't bother coming over, and then feel huge guild and have the fear of being 'punished' hang over me, or tell her the truth and hope that she understands and chooses not to come. (For me the worst thing that could happen would be that I tell her the truth and she is offended, or that she decides that as she feels better now, there is no need to worry and she will come over anyway, which will mean I can't get out of it, and I will have to suffer the massive anxiety which will happen however healthy she now feels).

I feel so guilty, but am trying not to beat myself up over it, (which I have a propensity to do).

Any advice greatly appreciated.