All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.
It is impermissible for you to go and take your sister from her husband's house merely because she said that she could not take it anymore. It is prohibited for the woman to leave her husband's house without his permission. Please, refer to fatwas 90507 and 99860.
If you take her from her husband's house without his permission – without a valid reason for that – you bear a sin for it, because you are thus helping her commit a violation of the Shariah. Allah, the Exalted, says (what means): {And cooperate in righteousness and piety, but do not cooperate in sin and aggression. And fear Allah; indeed, Allah is severe in penalty.} [Quran 5:2]
However, if you find that she is greatly harmed by her husband and that he physically abuses her, for instance, then she has the right to leave without his permission and you are allowed to go and take her.
Mughni Al-Muhtaaj, pertaining to the valid reasons for the woman to go out without her husband's permission, reads, "That the one who loaned the house drives her out or one who transgresses against her or threatens to beat her and she leaves the house out of fear for her safety; going out in this case is not considered nushooz (recalcitrance against the husband)..."
Allah knows best.