What is the ruling of Islam regarding someone who takes his sister's money and does not return it, despite being able to do so, and the sister refuses to forgive him, especially since she bore the burden of repaying the debt?

Indebtedness for a legitimate need is not blameworthy if the borrower intends to repay it. Otherwise, he is sinful and unjust, and it is permissible to complain about him to the judge. Debt is a grave matter that should not be taken lightly. "The delay of a rich person (in paying his debt) is injustice." The soul of a believer is tied to his debt until it is paid off. "Whoever takes people's money with the intention of destroying it, Allah will destroy him." The Prophet, peace be upon him, would not pray over a debtor until his debt was paid off. Whoever ponders these narrations will find it incumbent upon him not to borrow except out of necessity and to hasten to repay. [There is a balance] between demanding your right and forgiving your brother, knowing that forgiving an unjust person has great merit and preserves ties of kinship.