Divorce is not generally prohibited in all cases. It is, sometimes, obligatory or at least recommended at certain cases. Allah says in the Quran what means, “Divorce is twice. Then, either keep [her] in an acceptable manner or release [her] with good treatment. And it is not lawful for you to take anything of what you have given them unless both fear that they will not be able to keep [within] the limits of Allah . But if you fear that they will not keep [within] the limits of Allah, then there is no blame upon either of them concerning that by which she ransoms herself. These are the limits of Allah, so do not transgress them. And whoever transgresses the limits of Allah – it is those who are the wrongdoers.”  (Al-Baqarah 2:229)

So, when both parties or one of them is not able to fulfill the duties and obligations due on him or them towards others, one or both may refer to divorce.

It is recommended in this case. I suggest if the sister can be patient and endue living with the husband, Allah is going to reward her. If she makes sure she is not able to bear more and this will affect her in away she cannot fulfill the duties obligatory on her, she can ask for divorce.

Allah Almighty knows best.