All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
There is no doubt that being kind and dutiful to the parents is one of the best acts of worship by which a person gets closer to Allaah, and it is an emphatic obligation for a solvent son to spend on his parents if they are poor. Ibn al-Munthir said, “The scholars agreed in consensus that spending on the poor parents who do not have any means of earning and who do not have any money is an obligation from the money of the child.”
However, if the child is poor, then he is not obliged to spend on his parents or on one of them or on other relatives; Ibn Qudaamah said:
“There are three conditions for the obligation of spending: firstly, the parents must be poor, have no money, and have no earning to suffice themselves from the spending of others on them. However, if they are solvent and have money, or if they have a means of earning to suffice themselves, then their children are not obliged to spend on them, because the spending is an obligation as a way of support, and a solvent person does not need support. Secondly, the person who is obliged to spend on his parents must have money to spend on them, money that is in surplus of spending on his own self, whether this is from his money or from his earnings. As regards someone who does not have anything in surplus, then he is not obliged to do anything, as Jaabir narrated that the Prophet said, 'If any one of you is poor, then he should start by [spending on] himself, then if he has something in excess, then on his family [wife and children], and if he has something in excess, then he should spend on his relatives.' Another narration reads: 'Start by yourself and then on whomever you are obliged to spend on.' Moreover, Abu Hurayrah narrated that a man came to the Prophet and said, “O Messenger of Allaah, I have one dinaar.” The Prophet told him, “Spend it in charity on yourself.” The man said, “I have another dinaar.” The Prophet said to him, “Spend it in charity on your children.” The man again said, “I have another dinaar.” The Prophet said to him, “Spend it in charity on your wife.” Again, the man said, “I have another dinaar.” The Prophet said to him, “Spend it in charity on your servant.” The man said, “I have another dinaar.” The Prophet said to him, “You know best how to spend it.” [Abu Daawood] Since it is a support, a person who is in need is not obliged to give it, like zakah. Thirdly, the person who spends should be an heir; as Allaah says (what means): {And upon the [father's] heir is [a duty] like that [of the father].} [Quran 2:233]."
Based on the above, you are not sinful as long as you are unable to spend.
Allaah knows best.