If you are really overweight and truly need to be on a diet, then taking care of yourself is your first priority. Your health and wellbeing comes before what anyone else thinks of you. If your fiance doesn't approve, try really explaining to him how much this means to you. Maybe show him some articles about why this is a healthy option, or bring him with you to the doctor to talk about it. If you are gonna spend the rest of your life with this person, you have to be willing to stand up for yourself and share ideas with him and talk things out.
If his family doesn't approve, well, tough you-know-what. It's nice to get along with your in-laws, but they are not your parents and you're not 12. Your life is your life and if they are the rude type that always wants to criticize, better start learning now, how to let it roll off your back.
I hope your fiance is able to come around and help you make the decision you need to make. My partner was extremely skeptical, so I ordered a few extra packs and had him try some. Once he realized they are "real food" and saw all the vitamins listed on the back of the packets, he accepted the whole thing.