C.W.
J.,
you are so young your step son probably has a problem seeing you as an authority figure.
I do not know if I have good advice, but I can share my experience with you. I was about 13 years old when I met my stepmother. She NEVER tried to mother me, that would have created competition between she and my mother. She was more of an adult friend, maybe an aunt. I am 40 now and still have a great relationship with her. In my opinion it is so hard to discipline our own children that we should not be required to discipline others'.
Talk to your fiance alone and ask him to take charge of his son, so that you can be a positive influence rather than the discipliner.
As far as the nutritional issues, I agree with you. I am too very conscious about what enters my body and my children's. You should set the rules on what is allowed in your house and what is not. If your step-son wants to eat junk at his aunt's house, so be it, but he should not bring it to your house to set a bad example for your daughter. But here too this rule should be communicated to him by his father not by you.
One more thing: it is ok for your step-son to not accept you as a discipliner, but it is not ok to disrespect you. This should be discussed by the three of you.
I hope this can help.
C.