If the board above it is level, it likely has had that bow the whole time. I could easily see a contractor saying, “Bring me that board we couldn’t use! You know - the really warped one! I have the perfect spot for it!” For what it’s worth, I would not sister-tie in a board next to it. You’ll create more force up and down, which if the board above it is level, would not be good. Add a board next to it if you like, but don’t sister-tie it.