Is the questioner considered a sinner after repenting from theft and repaying the stolen amount from his sister's money, which she forgave him for, knowing that he did not tell her the truth of the matter, but rather said that he owed a debt?

Praise be to Allah who has guided you and enabled you to repent. There is no blame upon you for informing your sister that the money is a debt upon you, and you are not obliged to inform her about the theft. In fact, you should not do so. Conceal yourself with Allah's concealment and praise Him for your repentance, for Allah the Almighty said: "Then He turned to them that they might repent." And in the Hadith: "Avoid these abominations that Allah has forbidden. So whoever is afflicted with any of them, let him conceal himself with the concealment of Allah the Almighty, and let him repent to Allah the Almighty."