As has been mentioned, doctors suspect tha Lupus can have a genetic link. If your family has some history of auto-immune issues (of which there are over 60 possibilties), then your chances are greater for developing one yourself. So, as has been sugessted, it might be a good idea for your sister to get tested for auto-immune issues.
I want to welcome you to our family. We are all here to help you in any way that we can.
Peace and Blessings