I agree with Missy Krissy that letting your family know that you want to lose weight is important. Do, however, expect them not to listen completely. In my experience, I have found that most people have tried to guilt me into eating sweets and cakes (for b'day parties etc) on more than one occasion, even though I explain every time that I'm on a diet. When this happens, you just have to put your foot down and say no. Walk away if you have to.
It might take a while for some people to understand that dieting is important to you, but eventually they will get the hint