We used to be out to only friends until our first gf passed away. Then my mom found out because I obviously couldn't hide my grief. When my mom found out, we didn't speak for 3mos.
Until my 18 (at the time) year old sister said to my mother that what my wife and I did in our life was our business. She also told my mother that she was going to have a relationship with me and my wife(s) regardless. (Still tear up when I think about that) After that my mom didn't understand, and still doesn't, but she wanted a relationship with her son more than not. She just asked that I tell her ahead of time so that it wasn't a surprise.
Since then, my entire family (with the exception of my 88 year old Puertorican, REAL TRADITIONAL grandmother...LOL) knows about our lifestyle. When we get invites, it's addressed to all three of us. I have to admit, we were very lucky that everyone took it well. But, it also helped to have an I don't give a f*ck attitude. They realized that we're going to do what we want no matter what. (I think I got that back when I was 16 and realized that I preferred bigger women and wasn't going to let what my friends say stop me from being with women that I preferred!!)