Sorry to hear that you are in this situation and hopefully there is hope for everyone to have a restful/restorative sleep.

I am the one who talks/yells/snores in bed. My partner has yelled at me in the middle of the night and has left the bed as well.

Ear plugs worn too often will hurt your ears and taking a sleep aid every night isn't a solution for you.

Sister has to realize she has a problem and should see a doctor.
She may have a deviated septum which can be fixed.
She may have allergies, ect...
You said she sniffs during the day sinuses issues, most likely.
ONLY a DOCTOR can say!

Try to approach it with her like a project to work on together.
You need your sleep. Look into some of the things I mentioned and bring that information to your parents.
Good Luck