The Inheritance Games by Jennifer Lynn Barnes

She came from nothing.
Avery has a plan: keep her head down, work hard for a better future. Then an eccentric billionaire dies, leaving her almost his entire fortune. And no one, least of all Avery, knows why.

They had everything.
Now she must move into the mansion she's inherited. It's filled with secrets and codes, and the old man's surviving relatives - a family hell-bent on discovering why Avery got 'their' money.

Now there's only one rule: winner takes all.
Soon she is caught in a deadly game that everyone in this strange family is playing.But just how far will they go to keep their fortune?