You are already getting lots of good advice here, but I just wanted to say, as the daughter of two selfish-as-hell narcissists myself, that your personal value isn't determined by your usefulness to your parents. Sure, your brother might give them money/time/effort and they might reward him with their approval, but this is infantilising and should have ended long ago when he was about ten years old. An adult should not define their value based on parental approval. If you do this, your life will never be your own and everything you have or do will be determined by them and consumed by them.

Also, I'm sure you know this, but I feel you've been trained to jump in and solve their problems every time they complain, and this is why you are so quick to make offers you later regret. Maybe next time they moan about a situation they got themselves into, stop and consider first, if it's reasonable for them to expect you to fix their problems. They are probably a lot less helpless than they appear, and possibly a tad too manipulative of your good will and desire to be a 'good daughter.'