I'm looking for a book I read as a teen, it was about a boy (his sister became a main character as well) (modern day) who discovered the power of runes as seen in Norse mythology.
Casey indicated in the comments that they believe the book to be The Runestone Saga, by Chris Humphreys. The first book, The Fetch, published in 2006, has Sky and his cousin finding Norwegian runes belonging to their grandfather.
In the attic, in an old sea chest, secreted away, are a mysterious journal and a set of runes: 24 stones that will change Sky's life forever.
When Sky and his cousin Kristin find their Norwegian grandfather's runes, Sky feels like the wait is over, like he can now release the breath he wasn't even aware he'd been holding. But when he lays out the stones in a runecast they find in Sigurd's journal, he is catapulted into a world filled with more possibilities and more danger than he ever imagined.
The Kirkus Review of the book provides a bit more detail about the supernatural and time travel aspects of the book.
A boy is drawn into the violent past of his grandfather and Viking ancestors. Sky’s not looking forward to a summer with his estranged cousin Kristin, until they find Grandfather Sigurd’s runes in an old chest in the attic. Runecasting leads Sky to animal shapeshifting, dangerous haunts and visions of going a-Viking with his berserker forefather, Bjorn. Sky and Kristin run off to Norway on a mysterious quest, where they discover that Sigurd is neither so dead, nor so benevolent, as they’d originally suspected. Sky’s lessons in unexpectedly potent violence lead him to some hard decisions. The cousins slog through overwritten prose (though some historical Viking scenes are brought beautifully to life with language reminiscent of Anglo-Saxon alliterative rhythms). Despite pacing, a compelling adventure that asks hard questions of its characters.