Is it permissible to borrow money from a non-Muslim sister, an amount which she withdraws from the bank with interest and commits to repaying herself, on the condition that the original amount is returned to her without any increase?

Dealing with usury is strictly prohibited and is one of the major sins. There is no difference in this regard between a believer and a disbeliever; as disbelievers are addressed by the ancillary rules of Sharia according to the most authoritative opinion, thus what is forbidden for Muslims is also forbidden for them. This is indicated by the Almighty's saying: (What landed you in Saqar? They will say: We were not of those who prayed, nor did we feed the poor) (Al-Muddaththir: 42-44).

Moreover, usury was also forbidden for them in their original law, as the Almighty said: (And their taking of usury while they had been forbidden from it) (An-Nisa: 161).

Therefore, it is not permissible for you to direct your sister, assist her, or approve of her borrowing with usury; because that is considered cooperation in sin and aggression.

However, if she has borrowed with usury without your direction, or despite your refusal, or before you knew of its prohibition, then it is permissible for you to borrow from her a good loan (without interest); because she owns the money through the usurious loan and she is permitted to dispose of it, even though she bears the sin of engaging in usurious dealings.