How should a wife act towards her husband who cheated on her with her sister, and should she confront him?

If the actions of the husband and the wife's sister, which are impermissible, are proven, then the wife should advise her husband kindly, reminding him of Allah and His punishment, and that his actions could lead him to indecency. The same applies to the sister. It is permissible to threaten them with informing someone who has authority over them, so that this may deter them. However, caution must be exercised against acting on accusations, for the default state of Muslims is innocence until proven otherwise.