Is zakat still obligatory for a sister who owns 20,000 EGP and jewelry worth 15,000 EGP, while her husband earns a salary of 2,500 EGP and an additional income of 800 EGP? And would we be sinful for having given her zakat previously if she was not eligible?

The limit of the poor person who is eligible for Zakat has been previously clarified. If the sister possesses wealth exceeding her needs, she is not among those eligible for Zakat, and she must spend from her own wealth. If her wealth is depleted and she becomes poor, then it is permissible for her to receive Zakat.

As for the Zakat that was previously paid to the sister: If you believed her to be poor, but it became clear that she was wealthy: There is a difference of opinion regarding the obligation of making up the payment. The view that it is sufficient [without making up] is strong in this case, but re-issuing it is more preferable. If you were doubtful about her eligibility at the time of payment: You must re-issue that Zakat because your obligation has not been absolved.