I would not tell your father that you cannot stand your cousins.  However, you SHOULD tell him that you DO NOT want to give them any of your silkies since it is obvious that they plan to eat them, and that the thought of that horrifies you; that these are your pets that you have handraised and loved.  Let him know that if he really wants to cut down the size of the flock, that you are willing to sell or give some of them away to homes where they will be pets, not food.
Do not bring up any of the other stuff.  It is beside the point, and will lessen the impact of your plea to keep your birds.
Clothes that you have outgrown or furniture that you no longer use is really your parent's call, although if there is something really special, it is okay to mention that you really want ___ as a keepsake.  Having their kids live with y'all for high school and financing their grandmother's visits (is she your relative, too?) is really something that your parents will decide.
I expect the comment about your name was said teasingly.  As for what they spend their money on, well, it is THEIR money, and since they are the ones who have earned it, are allowed to make any choices they want on how to spend it.  Most parents spend money on thier kids.  Your parents spend money buying your chicken food, for example.