Is an brother who worked on agricultural land for 15 years for half its income entitled to claim ownership of half the land, knowing that the land is still a debt to the state and there was no prior agreement on that? And what do the Shari'ah and the law say regarding this matter?

The questioner did not clarify how he acquired the land, whether it was shared or private, belonging to him or someone else, or whether it was state property. Seizing others' property unjustly is forbidden, as Allah Almighty says: "And do not consume one another's property unjustly" (Quran 2:188), and the Prophet, peace and blessings be upon him, said: "Indeed, your blood, your properties, and your honor are sacred to you," and "Whoever unjustly takes a span of land will have it encircled around his neck from seven earths." Therefore, the question needs clarification to be answered.