Is it permissible for a brother to become financially independent of his siblings, with whom he shares economic resources, given that he bore the costs of his own marriage without anyone else sharing his burden, and that he is required to contribute to the marriages of his older brother’s disobedient children?

What you mentioned about sharing economic resources is a good custom that strengthens affection and family ties. However, it is not obligatory, nor is it necessary to continue it. There is no harm if everyone who wishes to be independent with their wealth does so. Unless you were previously dependent on your brothers, it is good to reward them for supporting you, but that is not obligatory upon you as long as they spent on you voluntarily. Similarly, you are not obligated to contribute to the marriage of your nephew's children, but if you do so seeking the pleasure of Allah, you will be rewarded for it.