What is the ruling of Islamic law regarding a father's refusal to lend money to his sister and her husband for the purchase of land, based on the request of his parents, considering that he believes it conflicts with his family's interests and future needs? And does he sin by not responding to his parents' request, especially since his mother is insistent upon it?

If you can lend money to your sister, it is a good and recommended act. The reward is even greater if she is a blood relative, in addition to pleasing your parents.

However, lending to her is not obligatory, and you are not required to obey your parents if they command you to do so if it results in harm to you, such as a long repayment period. Obedience to parents is not required in matters that cause harm to the child.

Therefore, you do not incur sin by disagreeing in this situation. However, strive to appease your parents, especially your mother, to avoid their anger.