How to differentiate between business and special interests if your counter negotiator is a friend or relative?

I think here I have an advantage as I already know the negotiator. I have to consider my relationship with them and find out ways my business isn't interrupted in any way and relationship also flourishes and I think that's what happens in every negotiation. And presently building relationships in business is needless to say very important and both the parties have to be in win-win situation along with both business as well as special interests