You can set the sprite's scale to a negative value.
E.g. to flip the sprite horizontally (X axis), set its scale to (-1, 1):
sprite.setTextureRect(sf::IntRect(50*ActualXframe,50*ActualYframe,50,50));
if(isFlipped)
sprite.setScale(sf::Vector2f(-1, 1));
Note that this will require the sprite's origin to be in the center. The flipped sprite will appear 50 pixels off to the left if you do not correct the origin (which is the top left corner by default).