At a family gathering make the food brought by the guests available to everyone; however, you can eat the healthy dish you made for the occasion. (No one will notice.)  At the end of the party, box up the leftovers and send them home with their owners.  If they refuse, throw them out when they leave. (Alternately, you can invite them for a repeat meal a day later.)  At the gathering, look for anyone who has switched to healthy eating, start a conversation, and see where it leads...  When you are you are given a gift of food you don't want but are not at a party, accept it graciously and regift it or throw it away.  Depending upon the nature of your relationship with the giver, you can say you are staying away from that food for health reasons.