Whoa. Send that e-mail and you'll pay for it -- and pay and pay. She would love the attention this gives her and would parse every word of it for things to argue about with you all during the camp. She'll have it memorized and be ready to thump you with it the whole time. If you want distance from her, create it yourself while there.
Immediately, I would contact the person in camp administration, the one whom you say knows the situation, and ask that person if he can help you out by ensuring that your sister does not end up registered for any sessions, KP duties, etc., for which you or other family members are registered. Your e-mail draft mentions that she should not sign up for sessions for which the rest of you are signed up, but how would she know what you're signed up for? Are those lists made available to every participant in advance? (I sure hope not; that sounds like a bad way to run any camp, frankly.) It's asking a lot of your camp contact but maybe you can offer to do extra things to help out when there -- extra administrative help in the office, for instance -- to make up for the favor of ensuring she's not with you. He may have to tell her that "the session's full" when it isn't -- is he willing to do that , at this Christian camp?
If your sister calls the camp to ask "What events are my relatives signed up for?" I would ensure that the camp staff knows to reply, "Sorry, we don't give out that information about anyone." If they DO do that -- well, the camp has some serious privacy issues that are beyond this situation!
I would NOT send this e-mail to her or contact her at all prior to camp; she will eat up the attention if you do that. In fact, starve her for attention -- even the "negative attention" of arguing with her or being rude to her -- the entire time. She is at an event with you or an activity? Focus your eyes and attention on that, not on her. Say a cool hello and goodbye every time -- do not just ignore her existence or she will have that to brood and complain over, so do greet her and answer the most basic questions with only the briefest basics ("Yes, Jenny went to X session yesterday. No, I'm not sure what session she has today. OK, the speaker's starting and I said I'd sit with Friend over there, so see you tomorrow." Depart.)
Mealtime is harder if it's family-style group tables. You can all agree to be cool but still speak to her, and not let her get to any of you, she will grow bored with the lack of drama and back off, one hopes.
But ultimately this sounds so stressful that I would not go. Yes, your husband is a speaker, but is it really critical to his role that all the rest of you be seen there with him? Surely you know what he's going to say. He would like your support, I'm sure, but he also surely would not want you have day after day of this stress, possibly capped off by some catastrophic argument at the end of camp. You would get nothing out of this whole time except stress. I'd let him go, wish him Godspeed, and spend the vacation time somewhere sister-free and enjoyable.