First explain it is a afull thing, in the eyes in wicca, curseing the family and i feel bad for you to be related to him. Now a simple "return the curse to the sender spell" also, if it were me i would have the words "and multiply it's strength by 3" somewhere in the spell, just to let him know it has been sent back. Goodluck
(ps. when his is stricken with a curse his ego will probally shatter"