I love how my coworker can literally sleep on the job and get away with it 😒

Are you allowed to film with your phone in the workplace?
You can get her fired and being the manager's sister will not save her.
Or tell her she has to give you half of her wages from now on or you will upload the video of her asleep to YouTube.
Tell the manager if they do not agree to give you an amazing performance review and a pay rise you will upload the video to YouTube.

I've got a feeling you nailed it right there. Must be infuriating.

God helps those who help themselves answers to such craps within you ..now how to go about it without un ruffling too many feathers is the key ...you have to slowly gather evidence of payslips..videos..if possible CCTV footage ..and at least get five six co workers with you n thats should nail some good dollars

She may have health issues or be on medication that makes her tired. I can understand you being upset though. I have fallen asleep at my desk and been very embarassed about it.

I'd probably video this on my phone, religiously, keep a folder with "coworker sleeping again"
Basically ignore her..
Do amazing at your job, then go to the "boss" the first chance you get... And ask for a raise or overtime as you've been covering for the sleeping 'beauty' for months..