That really is weird. Is it still going on?
Ya, it is weird that your mom chooses to sleep nude with her 20 year old daughter. Even with down syndrome, people deserve respect and privacy. And they can go on to have a productive life. Are there any other caregiver's in the picture? And guess what . . . what your mom needs to be considering is that your sister's care will eventually go to someone else which is probably you.
I have a good friend dealing with this now. Her father has cancer and her mother is aging all of a sudden at a very rapid rate. Her brother has cognitive deficits due to severe autism. She is being named his guardian this month due to the health concerns of her parents who had otherwise been doing the job. I'd talk to your mom about this first. What are her long range plans for your sister. If it involves depending on you down the road, you are in a better position to say that's not a healthy situation.
Voice your concerns carefully. But yes, voice them.