What is the ruling of Islamic law regarding a sister's objection to her younger brother taking possession of a portion of land that his parents bought for him before the division of the inheritance?

First: If the cultivation of the land, the sale of its produce, and the purchase of new land in your name were done with the explicit or implicit approval of your siblings, then the land is yours. Your mother's preference for you due to your need, over your married siblings, is considered legitimate in Islamic law.

Second: If this was done without your siblings' consent, or if you continued cultivating after your sister objected, then this action is forbidden and considered usurpation. In this case, the crops and what resulted from them belong to you and your mother, and you both must pay the rent for the land for the duration of the cultivation and distribute it among all heirs according to their share, or reconcile with them to your mutual satisfaction.