Is it considered a sin to stop paying the usual sponsorship amount to relatives who receive a sufficient pension, and are these individuals deserving of sponsorship?

Sponsoring an orphan is a great deed for which its doer is rewarded, as the Prophet, peace and blessings be upon him, said: "I and the sponsor of an orphan will be in Paradise like this." If you donate money to orphans, this is a righteous deed for which you will be rewarded, and you are not obligated to continue it. The stipend orphans receive is considered sponsorship for them if it suffices their needs. It is preferable not to discontinue good deeds, especially if the stipend is insufficient to meet their needs, for Allah Almighty says: "And whatever good you do - indeed, Allah is Knowing of it" and "And whatever good you put forth for yourselves - you will find it with Allah. It is better and greater in reward."