I would call her tell her that although you haven't discussed it with her, a major part of your ocd revolves around germs and contamination, and that even if she is feeling better, for your own peace of mind you would like to rain check the visit. And be clear that that is final, don't put yourself in that position without having covered dealing with these situations with your therapist. If she doesn't understand or thinks you are out of order, or is offended in anyway, thats on her, not you. you have to look after you in these situations, not worry about a minor offense to your sister. And id love to hope she would understand, and that this would maybe open the door to you being able to discuss your ocd with her, I don't know her so I'm agai only hoping that once she realises how badly the ocd can effect, that she would be supportive, but if shes not, its ok, just don't let yourself be pushed into a distressing situation when you aren't fully equipt to handle it. I have only recently been diagnosed with ocd, and not started therapy yet, and mine revolves around control rather than germs etc, but i have offered my opinion anyways, and im hoping whatever you chose to do about the situation, you continue to find therapy a help.