First, let's not stress about this event; I can assure you that 90% of the dads out there absolutely do not care. So if you can just come up with "something", that will be good enough.
Second, fathers always buy what they need, they probably have already all the stuff that they consider as a necessity so... I advice you to go for the exact opposite. Think about buying something that your dad did not put on his "must-have-list"...but still go for something that he will certainly enjoy.
Last, think about your dad first, do not listen to what your mother and brother wants him to have, think about HIM. At the end, that's HIS day!
If daddy likes sports
-> New golf bag/clubs (look for his favorite brand), tickets to a baseball game, new jersey of his favorite team, sports book, personalized baseball bat...
If daddy likes to party
-> Beer machine/ beer bucket, concert tickets, cookbook, whiskey glasses with name engraved...
If daddy likes to stay home
-> Apple Tv (best invention ever if you like to watch tv ..A LOT), personalized robe/coffee cup/ ipad cover...
If daddy likes you to be creative
-> make him a key-chain, cook breakfast for him, paint a canvas, do a photo-collage...
Hope this helps and your dad will have a great day ! :)