I have a sister who is feckless with money. She asked for financial help from our mother when she was in need. Our mother was good enough to tell me about the request and asked if I minded (I didn't). She proposed giving me a matching amount of money, although I didn't need it.
However, after our mother passed away, I came across another begging letter from my sister to our mother and I have no knowledge if a gift was made at that time, or not - or indeed any others. I should mention that my sister left for another country and did nothing to support our mother in her extremely frail state of health for her last fifteen years.
It is not the act of giving money but the background behind it that (to me) is the problem. Some people need or manage money better than others but if there is a constant stream of financial help there could be a perception that one person is being left out. Of course, my mother was free to do what she wished with her money but there was a niggling sense of unease when I found the correspondence relating to a second gift.
I would suggest you speak to your parents, perhaps suggesting that if possible, a gift be made now to your brother and this amount be deducted from their estate when they pass away, if they do not wish - for whatever reason - to give equal amounts now.
Please do not let this fester as many years later, despite a good relationship with my sister, if I think about this quandary, it still rankles! Perhaps I am a very shallow person!