"I'm very excited," he said of the couple's upcoming wedding. "I think Sophie is an incredible addition to our family. She's like a sister to me already."
Nick said Turner, who plays Sansa Stark on Game of Thrones, keeps a tight lid on the show's plot despite their close relationship.
"She's very good about her confidentiality agreement," he said. "So is [Joe]. He's not allowed to say anything either. But also, I don't really want it to be spoiled. I'm kind of a fan seeing it as it happens."
Nick previously told People his brother's relationship with Turner seemed "meant to be."
"I am very close with Joe," the star said. "It was pretty clear to me early on that this relationship was one that was sort of meant to be, in a sense."
"Their connection's beautiful and their support system with each other is incredible, and everything that you hope to find in your soulmate," he added. "I'm really happy for them."
Joe and Turner got engaged in October after nearly a year of dating. The actress said in the May issue of Marie Claire that it's "lovely" to have found her person.