Paying Zakah to indebted sister

My sister is not married; she works and earns her own living. However, she doesn't have a house of her own. Some time ago she took a bank loan (Riba) and purchased a house. Now she feels sorry for that, and asked God for forgiveness. Her loan and living expenses are more than her salary can handle. So can I pay my Zakah to her or not?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

It is intended that Zakah should be given only to those recipients who are stated in the Qur'an. So, if your sister is one of these there is no harm for you to give her a portion of your Zakah. But it is unlawful to give it to her if she will invest it in Haram interest.
This is not lawful because Allah Says (interpretation of meaning): {… but do not help one another in sin and transgression. …} [5: 2].
The evidence for giving Zakah to this sister is that she is indebted and the indebted people are entitled for Zakah. Allah Says (interpretation of meaning): {… and for those in debt, …} [9: 60].
But she should repent from Riba dealings and if she can pay only the capital, she must do so and should avoid the interest if possible.
Allah knows best.