I read this as an audiobook when I was a kid. From memory it was geared toward Middle School aged kids. Two brothers I think that they were twins. Lived in an orphanage and the orphanage was in like a landfill. Or maybe that was just a specific word they used to describe the place.

The main character's brother was into art and he would cut the hair of the other kids to make his paint brushes. One brother stars and his legs in the shape of circles and the other had scars that were sharp and angular like triangles. When they sat next to each other the scars lined up. Later in the book they used the scars on their legs to find their way through a castle or some type of tower.

I can't remember how or why they left the orphanage and somehow ended up on a ship that took him to another world a good part of the book is them being on this ship. It's a series if I remember they're being like five or more books. A heavy water theme I definitely remember. At the end of the book I think the artist brother stays on an island and the other stays on a ship to sail around.