@tanguatlay,
I posted then pulled this reply as I'm unsure about the first comma, still am ?
After over 60 years of mangling the English language, my take would be
Without commas it is unambiguous, you have a sister called Mary and she is clever
After inserting the 1st comma
Mary is not necessarily your sister, you could be addressing another sister(Jane) and telling Jane, Mary is clever
After placing the 2nd comma
"is clever" becomes a question, so your asking Jane if Mary is clever