Mommy Friends :) Need Advice

The parent forums I'm in are deader then dead for the last month or so, so I was hoping I could get better advice in a journal!!
 
The issue is my sister and my nephew.  I am not a health nut, by any means.  We try to eat enough fruit and vegetables in a day but I'm not opposed to feeding my daughter KD or Alphaghettis for lunch.  We also consume more then a healthy amount of sugar some days! 
 
I care for Julian (my nephew) for nine hours a day, five hours a week.  When he first came to me he didn't eat anything that wasn't processed.  No fruit, no vegetables and no meat that wasn't a hot dog.  He's three and a half by the way.  Over the past four months I have gotten him to try pickles and beans and try AND like strawberries, red apples, bananas, carrots, peaches, pears, corn and broccoli (though that ones in the air still).  He eats porkchops, chicken breasts, steak and porkloin.  He refuses to eat any type of sauce....literally, none, so most of these are just lightly spiced.  My sister is estatic he's eating so much and she's been thrilled that he eats some of it (not all of it) for her.
 
But lately he has been showing up at my house in the morning eating the most HORRID things!  One day he showed up at 8am with a can of half drank rootbeer.  Today he showed up at 9am eating half of a chocolate Easter bunny!  He shows up with poptarts, chocolate bars, chips, bags of sugary cereal.  I immediatley take it from him and offer something else up.  We eat sugary cereals, but not daily and not without milk and some fruit. 
 
When I broach it with my sister she gets defensive and tells me that I'm attacking her parenting style and that she's making him eat the other things at home like I suggested so why should she also have to fight with him in the morning?  She points out all my parenting faults and gets really upset even though I try to phrase it as nicely as possible. 
 
Should I just continue to take it from him the second he gets here and offer him healthy alternatives?  Or should I tell her he isn't allowed ot show up eating them here?  Or should I offer to feed him breakfast (though we can't really afford it and I already feed him two meals a day most days)?  I'm worried about my own daughters health daily, but I'm extra worried about his!!