Can you take him to a vet tomorrow? another one that actually knows about birds?
Cockatiels go down hill so fast, they can be fine one minute and gone the next, so it's not something to muck around with.
Has anything been sprayed nearby - perfume, fly spray, deodorant etc or has your sister used anything with teflon?
I hope he feels better soon, I'd still take him in the next day to be safe rather than sorry - please keep us updated.