Try sleeping without sheets. Many ex-smokers say they quit "cold turkey" and ripping the blanket off your bed is the same thing. If you can't handle that, you'll have to train yourself not to sleep with the sheets completely enveloping you by slowly pulling the sheets down each night. Start by exposing just the top of your head (the hair part). Once you can comfortably sleep like that, pull it down so that your eyes and nose are now visible. Once that's comfortable, pull the sheets down to your chin. And on and on until you have accomplished your goal.
You will need support from your family, also, so you may have to explain this to them. They may also have great advice on how to get over the fear. You will need a lot of positive reinforcement, so your little brother is ABSOLUTELY NOT ALLOWED TO SNEAK INTO YOUR ROOM WHILE YOU'RE SLEEPING AND SAY "BOO!" one night, or you'll forever be doomed to sleep with the bed sheets covering you completely.
Something else to consider: in ancient times, dead people were completely covered in a sheet and then buried in the Catacombs.