What is the ruling of Islamic law regarding a father's financial support for his adult son who is a student of knowledge, and regarding the financial support of brothers for their brother, and is it obligatory to repay what they have spent?

Your father must spend on you for your university studies, just as he did with your siblings. Your siblings are not obliged to spend on you; what they spent is considered a donation from them, and they do not have the right to reclaim it, unless they spent it with the intention of reclaiming it from you. This has no relation to the inheritance.