An 's is for singular nouns, while an s' is for plurals. When the shop first started out, both Fred and George ran the store, making it the Weasleys'. Then, well...
BUT EITHER WAY THE POST IS WRONG BECAUSE TWO YEARS AFTER RON BECAME AN AUROR HE QUIT BECAUSE HE WANTED TO HELP GEORGE WITH THE STORE JK ROWLING SAID IT HERSELF SO IT'S STILL RUN BY MORE THAN ONE WEASLEY AND IT'S STILL CALLED "WEASLEY AND WEASLEY" SOMETIMES AND THIS POST LIES IT IS FULL OF LIES AND TEARS AND WAS EXTREMELY UNNECESSARY AND WHY MUST THEY DO THIS TO US
BUT EITHER WAY THE POST IS WRONG BECAUSE TWO YEARS AFTER RON BECAME AN AUROR HE QUIT BECAUSE HE WANTED TO HELP GEORGE WITH THE STORE JK ROWLING SAID IT HERSELF SO IT'S STILL RUN BY MORE THAN ONE WEASLEY AND IT'S STILL CALLED "WEASLEY AND WEASLEY" SOMETIMES AND THIS POST LIES IT IS FULL OF LIES AND TEARS AND WAS EXTREMELY UNNECESSARY AND WHY MUST THEY DO THIS TO US