@shyevsa
@Hollow7F
Tabi (旅) means travel, trip, or journey. Tabitabi is simply "tabi" repeated twice for poetic purposes. Think of it as "multiple trips/journeys". If you remember, the first chapter was a trip to the Country of Magicians, while this chapter is about a trip to the Country of Flowers. So this story isn't about a single journey towards a final goal, it's about many different journeys that she'll undertake.
It doesn't really explain anything about the character, other than that she likes travelling a lot.
As for why she chose not to help the brother: she had no emotional attachment to him, and thus literally no reason to help.
Her first impression of him was that he's an asshole, so she had no intention of helping him in the first place. She got a bad conscience when she realized what had happened, and when she saw the two finally reunited, she probably imagined it'd be pointless to help him now. His sister's dead, which would probably drive him insane even if she cured him.
The Eastern girl was a different case. At least she made it look like an accident, and she was probably the only person to treat her nicely. And since Elaina agreed to teach her, the distance between them gradually shortened, to the point where Elaina could forgive her for stealing her brooch.