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.
In general, being kind to relatives, especially the needy ones, is among the great acts of worship by which one gets closer to Allaah. If these relatives are in need, then they have priority over others unless there is greater benefit in giving priority to others before them.
As regards your brother, if his situation is as you mentioned that he is extravagant and a spendthrift, then you should advise him and remind him of Allaah as he will have to answer to Him about how he spent his money. Indeed, the Prophet clarified to us that every person is going to be asked (on the Day of Judgement) about how he spent his money. [At-Tirmithi]
So your brother should be reminded that there are poor Muslims who are more in need for the money than him spending it on trivial things.
However, it is permissible to give him some money and helping him, but you should balance between giving him and spending on the needy and destitute people.
Allaah Knows best.