Would it help if you explained to your parents how keeping this a secret is affecting you? Perhaps they should understand how stress can make this illness much worse than it need be. Do you think your parents are open enough to listen to you?
It is just as important for your parents to be as educated and informed about Lupus as you are. If you like, we can suggest some books etc. that will help them to understand this disease. Let us know how we can help you.

Peace and Blessings
Saysusie