The Great Unexpected by Sharon Creech (2012) Joanna Cotler Books 

Naomi and her best friend, Lizzie, live in the town of Blackbird Tree. They’re both orphans living with people they’re not actually related to.  Naomi lives with Nula and Joe, who are kind-of her grandparents, but not really, and Lizzie lives with foster parents she thinks will adopt her.

Then there’s a boy who falls out of a tree and lands on Naomi.  His name is Finn, as is every fourth boy in Ireland.  But they’re not in Ireland.

Then there’s a woman in Ireland who is setting-up her revenge.  And it involves Nula and Naomi and Lizzie.