This sounds like you've circled back in the direction of traditional vanity publishing?
Do you imagine the entire market is friends and relatives? What isn't working with your current approach that you hope to solve with profit sharing? Unless you expect these books to be profitable sellers, and profit to be a motivator ... my first intuition is that if I wanted my Grandmother's story told, first paying a fee and then trying to get half that fee back when people like my uncle buy a copy... that doesn't make sense to me, maybe it does to other people. What problem are you hoping to solve with this sales approach? Are you hoping to motivate customers to get more involved in pushing sales once the books are published, lower up-front payments?
I wonder if a kick-start or pre-sales model might help if these are the issues you want to solve -- I'd pre-order a book about/by my Grandmother. It would let your clients tell their family about the book before they put money down, which would then mean some relative would get excited, and after that you'd have momentum.