I don’t know why your parents treat you and your siblings so unequally. You did make a good guess at the end of your letter. Since he doesn’t see your siblings very often, it could be that your father is trying to buy their love. But that doesn’t seem to explain the extent of the imbalance.
If you are complaining about how much he spends on your siblings, your father isn’t likely to be responsive. I wonder what would happen if you instead told him how much the difference in treatment hurts you or that you fear that he doesn’t love you. Perhaps if you opened up about your feelings, he would be more responsive. The same thing might help with your mother. Instead of focusing on the material items, talk to her about how both parents’ behavior makes you feel sad and unwanted. Your parents may be more willing to respond to you if you let yourself be vulnerable with them instead of angry.
I wish you well.