Defining and non-defining relative clauses

Read the sentences. Are they the same or different?
Ann’s sister who is a tennis champion visited her last weekend.
Has Ann got more than one sister?
YES
Ann’s sister, who is a tennis champion, visited her last weekend.
Has Ann got more than one sister?
NO

Defining relative clauses
They give ESSENTIAL INFORMATION
We DON’T USE COMMAS in them.
Ann’s sister who is a tennis champion visited her last weekend.
The table whose legs are broken is at the back of the classroom.

Non-defining relative clauses
They give EXTRA INFORMATION
We USE COMMAS in them
Ann’s sister, who is a tennis champion, visited her last week.
(We probably know that Ann has got only one sisiter)
Charles Dickens, whose novel Oliver Twist we read last term, was born two hundred years ago.
(We already know that Ch.Dickens wrote Oliver Twist)