You said you generally have a family lunch, do you usually do an invitation? If not, don't do one now, just get the cake and spread the word that day that later there will be cake and you'll let everyone know the sex of the twins. If you DO usually do an invitation for lunch (or if you just have to put it out there so everyone knows whose house the lunch is at and what time, etc.) then do like normal..."Hey everyone, we'll be in town this weekend and we'll have the family lunch at Aunt Martha's on Saturday about 11. Make sure you are there by 3:00 because we are going to annouce the gender of the twins!" Even what you said doesn't sound like a shower or fishing for gifts to me...you could also state point blank "no presents please", but some people WANT to bring presents and they DON'T want to ship them! Ha, ha!