What are the rights of younger siblings over his older siblings?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

There are rights required by the lineage that exists between brothers, and among these rights are the relations of kinship that Allah the Almighty has made obligatory to maintain and forbids severing. Allah the Almighty says (what means): {...And fear Allah, through Whom you demand (your mutual rights), and (do not cut the relations of) the wombs (kinship)...} [Quran 4: 1]

Among these rights is spending due to kinship relation, according to those jurists who held this opinion, and when it meets the conditions for its obligation. This is in addition to the general rights among Muslims, such as the elder’s mercy toward the young, as stated in the hadeeth: “He is not among us who does not have mercy on our young and respect our elders.” [Reported by Ahmad, Abu Dawood, and at-Tirmidhi]

Allah knows best.