Wow sorry, 「お姉ちゃん」is vocative. Try お姉ちゃん、れい君が勝ったって言ってた。Who did say 勝った? れい君 did.

This can be お姉ちゃんがれい君が勝ったって言ってた。 Note that 「言ってた」is left out, but we can understand the sentence should go like that way. The two が are left out as well.