Try thinking of this in a different way - you have been presented with a wonderful opportunity to develop a valuable life-skill: The Art of Saying "No!".
You cannot out-exercise bad eating habits, nor is it likely the family will change their own, so unless you run away from your job your only option (aside from giving in and gaining weight) is to not eat all those tasty treats.
(in my Sir David Attenborough voice)
From my observations of healthy thin people it's apparent that one of the key aspects of their behaviour is that when presented with the option of eating crap, they generally make the decision not to. This is a learned behaviour and something that they use in other non-food related areas of their life. This behaviour can be learned by anyone.
The term "deferred gratification" is one I think you should "Google" for. Use that with the knowledge that less sugar and saturated fat in your diet means less chance of heart attack and diabetes and cancer.
I could tell you to stop right now and don't give in to temptation, but would you listen? No. The only person you are likely to listen to is yourself. You know what to do.
Hmm, the only piece of advice I can suggest is to hold one of the items of unhealthy food in your hand and visualise its fat and sugar content coursing through your veins and depositing fat in your arteries, caking it like the fur on the inside of water pipes. Imaging the sugar mucking up your blood and organs. Imagine the pain of a heart attack and the regrets might have in your later life as you hobble around in misery and pain, popping pills and awaiting the next surgical operation. Future regrets that COULD have been negated by a different decision than the "easy" one of scoffing down lots of unhealthy food. You get one chance at life.