Is the money I receive from my parents considered unlawful (haram)? And is what I spend from it an injustice to my younger brother, given that I receive 200 riyals and he receives 100 riyals? Is his consent to this favoritism valid, considering he has not reached the age of discernment? And would it be considered a fair arrangement if my parents intend to give him the same amount when he grows up?

It is obligatory to be fair among children in gifts and endowments, but as for maintenance, each of them should be given according to their sufficiency. The maintenance of an adult differs from that of a child, and the expenses of a university student differ from those of a primary school student. Adjusting expenditure means giving each of them what they need. If all the expenditure is considered maintenance, i.e., the amount sufficient for each individual's needs, then fairness is not obligatory; rather, each individual is given according to their need. However, if the expenditure, or part of it, exceeds the need, then the excess is considered a gift, and fairness is required therein.

Preferential treatment in a gift is permissible if the one receiving less gives permission, and permission is only valid from someone who is legally competent (an adult, sane, and mature individual).