His wife torments his daughter from another wife, and he has a son from her, and he is confused as to how he should deal with her

Praise be to Allah.

It seems that the series of sufferings that you are facing with this woman will continue, and the list of her bad deeds will grow longer. Her bad treatment of your little girl will never know any limit; if your daughter was an animal it would not be permissible for her to do what she has done to her. 

We are certain that if your son from her were to suffer as your daughter is suffering because of not being able to go to the bathroom by himself, you would not see him being treated as badly as she is treating your daughter. This -- unfortunately -- happens a lot with the wives of fathers, and many, both male and female, suffer from this. 

What we advise you to do is: 

You must deal with the matter seriously and quickly, try to instil a sense of love, mercy and compassion in her heart, in the hope that she will stop what she is doing. If you succeed in doing that, then you will have achieved what you want and she will still be your wife. 

If you do not succeed, then you have no choice but divorce. We do not advise you to be hardhearted and harsh, because that will only make her more cruel towards your daughter, and she will find more ways to harm her, whilst concealing the evidence of her misdeeds. Hence if she does not respond to you and change her behaviour, and mend her ways in response to kindness and persuasion, there is no solution that will put an end to your suffering except divorce, sooner rather than later. You are aware of your daughter’s suffering, and you hear her cries, and you see the injury to her face and the burns on her leg and stomach, yet despite that you're still confused because you have a son from her. So how would it be if there were more children then?! But we will not hide anything from you, and we will tell you frankly that with a woman such as this, whom you have divorced twice yet it has not stopped her wrongdoing and aggression, there is no hope for much good in her, and it cannot be expected that she will be kind to your daughter, because mercy cannot be instilled by means of threats or warnings in the heart of one whom Allaah has deprived of it. 

If you think of divorcing her for a third time, or she sees signs of that in you, do not leave your daughter with her, rather leave her with one of your relatives or with trustworthy women among your neighbours or friends, because she might try to take revenge on her because of your divorcing her. We do not know how you are going to deal with the matter, because women vary, hence we are afraid that she may do something to your daughter. With such women there is no regret in separating from them. 

We ask Allaah to relieve your distress and worry, and to reward you for your calamity, and replace it with good from Him. 

And Allaah knows best.