My sister started obsessively making this loud whistling, breathing sounds with her mouth that sound like a wave. she says it's anxiety ?

Mindfulness: Suggest books on mindfulness as they are a wonderful way to alleviate anxiety without medication.

Obessive mouth noise: This is a problem that needs medical evaluation. There are many possibilities including obsessive disorders, tourette's syndrome, movement disorders etc. Call her doctor for an appt. To be evaluated for the cause.