Should you not be setting "leftIsPressed" to false if it isn't pressed, same for right. Are they being initialised? Chances are that they are randomly being assigned true (anything but zero) if not.
You don't actually need these booleans as they serve the same purpose as isKeyPressed (you can use that directly in playerControl). However, if you still want to use them, assign them directly thus:
leftIsPressed = sf::Keyboard::isKeyPressed(sf::Keyboard::Left);
rightIsPressed = sf::Keyboard::isKeyPressed(sf::Keyboard::Right);
If you
want them to be exclusive, you can simply add:
if (leftIsPressed)
rightIsPressed = false;