I haven't tried this but I just looked at the code and from what I see you want to use Sprite::SetTextureRect() and pass in a negative width/height to flip the sprite as well as add the width/height to the x and y.
sf::Texture t;
sf::Sprite s;
//Flip the sprite vertically using the entire texture
s.SetTextureRect(0, t.GetHeight(), 0, -t.GetHeight());