Which is better: helping one's parents perform Umrah, or helping a brother get married?

We see that what takes precedence here is what pleases the parents, so present the matter to them. Whatever they advise you to do is more worthy, for their righteousness and the fulfillment of their pleasure are superior, as the Prophet, peace and blessings be upon him, said: "Your mother, then your mother, then your mother, then your father," and his saying: "The pleasure of Allah is in the pleasure of the parent, and the displeasure of Allah is in the displeasure of the parent."

They might advise you to give money to your brother to help him with his marriage, or for the expenses of their Umrah/Hajj, and you will have a great reward for that.