I believe that it IS rude to do that at ANYONE'S home, and perhaps these sisters don't know any better.
I agree, you should walk over, gently close the refrigerator door and then ask if they need a snack.
My DD knows that there are certain foods for friends and certain foods for us. For example, juice is expensive so I don't serve juice to her and her friends. They can easily drink an entire $4 carton of orange juice!
I have foods for friends, I usually keep some frozen lemonade in the freezer so my daughter knows she can make lemonade for her and her friends without asking. She also knows she can make popcorn or share our chips. We always have mini carrots, celery and dips and peanut butter to share.
I also ALWAYS have a frozen pizza in the freezer as well, that she can make for her friends without asking. They can make PB & J sandwiches or toasted cheese, but lunchmeat is expensive and we save that for my husband's lunch.
I would be upset at any child that just walked in and opened our fridge. They all know to ask me or my DD for any type of snack! Still, I would curb the behavior as nicely as I could.