Done with uni, time to go home for summer - PLEASE HELP

So I've been doing pretty well the past few months, since I've been at uni I'm the one responsible for buying my food, cooking, etc. My flatmate has also been trying to change her life style around so we do our weekly shops together and encourage each other to eat healthy. However, next week I am due to go home for the summer. My family are not the healthiest of people and I have cut out everything processed in my diet. I enjoy eating clean but I think being back in a house with a lot of temptation around me and not being the one in charge of cooking and shopping is going to be difficult.
I have a young sister so there's always snacks and things in the house which my mum won't stop buying.
I also don't want to be rude, ungrateful or bratty by not eating the food she loving prepares for our family meals - "requesting" her to buy me lots of organic veggies, fruits, quinoa, etc. is not something that will go down well as we are a little tight on cash at the moment as it is, and weather we like it or not we all know a bag of organic apples is more expensive than a cookie.

So, I'm just really looking for some advice on how to go about this. I have a routine here, I have a gym 2 minutes away, and a buddy to stay accountable to - going home means a massive change in routine, no gym (they are all extremely expensive and I would have to catch 2 buses to my nearest one!!!) and a weekly "Friday take away night" with the fam.
I'm so excited to see them but so nervous about falling off the wagon that it's putting a downer on things.
I know if I offered to cook my younger sister would NOT eat any of the nutritious meals and my dad refuses anything without meat
Just looking for some advice please!