^hi, kindly posts your queries in the main forum so that other members may also give their legal advice.
(a) As to the obligation to support the 2nd Wife, considering that she is just a step mother, the eldest son has no obligation to give support to her BUT, since the truck is part of the conjugal property during her marriage to your father, thus, she can claim more than half of the ownership of the truck (1/2 is her conjugal share, and then her share to the other 1/2 together with her minor son and the step childred), which includes the profit derived from it.
(b) As to the obligation to support the Minor Child, said child has the right to claim legal support from his half-brothers/sisters, thus, the eldest son has obligation to give support to his half-brother/sister together of course with his other siblings BUT only if the mother has no financial capacity to support her child as it is her principal obligationt to support her child.
(c) As to Child Abuse, it will be a bit difficult to prove it considering that it is not the elder son's full and primary obligation to give financial support to the minor child as it is principally the obligation of the mother, only if she do not have the financial means that the half-brothers/sister must jointly support the child. Nonetheless, if the elder brother is the only one who has the financial capacity to support and he neglect the same, then his act may fall under child abuse.