I think you owe it to your niece to tell her how it is and take the chance to educate her on the realities of life. Having been a teacher for more than 20 years I find it really hard to work with the airy-fairy teachers who won't face up to the negative.
I also think your sister is using it as an excuse not to face her lack of support with your mum.
If on the other hand your niece is suffering from depression and needs positivity then she shouldn't be traveling alone or teaching..........