Serious diet question please help?

I am going to Lake Powell with a friend and will most likely be taking pics in a bikini I have been really good and healthy all summer but I am worried since her family will be there and they all love food and sugar that's bad for you that I will have some unflattering pics and a bad body image, and I don't want to seem uptight and refuse everything and it's going to be my 21st bday so lots of celebratory cake might not me off the diet and workout track...helpp