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
Question

                I got married two years after my divorce, and I told my second wife that I had a daughter, and I hoped that she would seek reward with Allaah by raising her, but I was shocked to find that she treats my daughter badly, and she torments her. On one occasion she made her fall to the ground, and she injured her face -- she was two years old -- and she always threatens her because the girl cannot control herself when she needs the bathroom. As a result of that, it generated hatred and resentment towards her in my heart, and I have divorced her twice. I have a son from her who is 1½  years old. I can no longer put up with her bad treatment of my daughter, especially since I am far away from my family, and my mother is elderly and does not have the strength to raise my daughter. Her mother left her with me when she was three months old. Today I came home and she said to me: I burnt your daughter's leg and stomach with a spoon that I heated on the fire, so that she will learn to go to the bathroom by herself and not -- excuse me -- relieve herself in other rooms. I am fed up and all of my life is worry and fear for my daughter, who is now 4½  years old. I can no longer trust this woman, but I'm thinking of this poor child (my son).

Please tell me what the Islamic teachings are and advise me, may Allaah bless you. I have suffered, and am mentally and physically ill as a result of this.
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.