40 Year Old Man Hitting An 18 Year Old..?

i was on a vacation with my girlfriend and her family.. ok so im real close to all of them.. so her moms brother came with us hes about 40 something years old.. we was fine for the past few days then the last day we was there my girlfriend got into an arguement.. and we pulled over at a gas station because we was all in different cars.. and i was talking to my girlfriend and he out of no where says "what did you say ************" and i havent said anything to him. and he was drinkin also.. so he gets in my face for about 10 seconds then hits me just a reminder.. im 18 years old and hes 40 something.. so i didnt know what to do.. either to hit him back or let it go.. because hes drunk.. so now i havent this really bad guilt that i should have hit him and makes me look like a .. someone please help me with this situation and be serious.. and tell me what you really think.. thanks everyone god bless..