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".
Thank you very much for the reply Ovidiu. I understand your point about a group of mixed gender being taken as masculine. However, isn't îi him or her, and hence singular, whereas bunicii is plural? I still do not see why it is not le meaning them.
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"
OK I didn't realise îi was plural. In that case, I can only assume there is an error in the basic rules listed under the lesson where it suggests that the plural is le for BOTH masculine and feminine. It now all makes perfect sense. Thank you for your patience. :-)
I do not understand this sentence. Is "Băiatul le vizitează pe bunicii lui." not correct? If the verb "a vizita" takes a direct object then shouldn't the accusative pronoun be them? îi seems to mean to him.
As an addendum the next sentence is "Noi le auzim pe surorile voastre." meaning "We hear your sisters." There must clearly be a difference in required structure between a vizita and a auzi...