It's your wedding. You're the main attraction. It's your day. A day which you've probably been planning in your mind for years. You should have the absolute privilege to have whomever you want to walk you down the aisle. Even though weddings are a touchy thing when it comes to relatives, you should have anyone you want. I think you're right about your bio-dad. I think he lost that privilege a long time ago. My natural instinct would be to try and keep relatives happy and ask the brother, but he has already declined. So again, ask a dear friend. Or even the step dad. He's been the only father figure in your life. Your decision. Best of luck and have a great wedding.