I've said a number of times here that I do
not rule out lending money to family, however I think you are describing a case where I would make no more loans.
I would also talk to Mom about brother's problem.
If you want to try to test your determination to make no more loans to him, just try
asking him for a loan similar to the biggest size he asks of you or your Mom, and see what his reaction is. Loving reciprocity? Guarded, stammering tight-fistedness? Compassionate scramble to help you however he possibly can? Embarrassed remorse for having taken, taken, taken with no ability to now lend a hand?