"Băiatul îi vizitează pe bunicii lui."
Translation:The boy visits his grandparents.
No, the difference is not between the verbs. It's just that when we refer to a group of people of mixed gender we tend to use the masculine form of the plural.
In this case you have a grandmother ("bunică") and a grandfather ("bunic"), but when talking about both as a group you'd use the masculine "bunici".
In your example "surori" means a group of female siblings which is why the feminine plural is used. But if the gender was mixed or unknown you'd likely use the masculine form.
"The siblings divided the inheritance between them" = "Frații și-au împărțit moștenirea între ei". "Frații" really means "the brothers".
The singular is "îl" for masculine and "o" for feminine:
"El îl vizitează pe bunicul lui" = "He visits his grandfather"
"El o vizitează pe bunica lui" = "He visits his grandmother"
The plural is "îi" for masculine and "le" for feminine:
"El îi vizitează pe bunicii lui" = "He visits his grandfathers/grandparents"
"El le vizitează pe bunicile lui" = "He visits his grandmothers"