Ok, cos I always like to fix things.
Suggest you and your sister, and maybe the bf depending on how serious she is about him, all do a first aid course together so that you all know, or more specificially, your sister knows - what an emergency looks and feels like. Eg collapse for no apparent reason (or stress caused by excitement ie pending date) is an emergency.
Repeat collapse - ambulance.
Hope it all works out for the best and she's ok or has something manageable.