Biana and Linh

Moments

Biana defends Linh by saying "Aw, [Linh] has awesome control."
And then later, "And while Tam managed to force Keefe into a few of the research slots, he gave most of them to Linh, not wanting her spending too much time underwater-- no matter how many times Biana swore he was underestimating his sister."
When they found Biana when her face was smashed into the mirrors, Linh was the one to think of Livvy.

Both have a brother or brothers
Both have already manifested
Both are part of the Black Swan
Both grew up in the Elvin world
Both are elves
They are both at Foxfire
They both attended Exillium
They both hate the Neverseen
They both speak the Enlightened Language
Both of their families are in the Nobility
They both hold the unpopular opinion that their world is getting messy, yet the elves deny it

Biana is a Vanisher and Linh is a Hydrokinetic
Biana has gone to Foxfire for all of her life while Linh has attended Exillium for a long time
The households they grew up in were vastly different (Biana's parents were happy with their children and their accomplishments while Linh's parents were ashamed of their children)
They attended different hemispheres at Exillium
Linh was at Exillium for much longer than Biana.