Be persistent. Don't be a jerk. This is family, for heaven's sake.
When I took over the family wine business, my dad and I argued about everything--changing the name of the store, building a website, hiring developers, giving out 401(k)s. We always argued.
Even our fight about moving the beer cooler took six months. It's a common theme for family businesses. But what always won for me was a combination of massive communication and obnoxious persistence.
I think that a lot of sons and daughters give up. You get yelled at, get argued with, get upset, and don't come back to the well. I never did that. I wore him down. My advice is: If you believe it's right for the business, then find four different angles.
Go in hard, go in soft, come from every angle, and you'll find your way in. And whatever you do, never make it personal. Start bringing up shit that has nothing to do with the business, stuff that happened at home a month ago, and you're derailed.