defining/non defining relative clause

What is the difference between these 2 sentences:

  1. My sister whose flat is in central London, is a charity worker. and
  2. My sister, whose flat is in central London, is a charity worker.
    I know that the 1st one is a defining clause and it is written without the use of commas but I refer to their meanings.
    Thank you for your time!

The first sentence is incorrect. Firstly, the comma should not be there (or you should have two commas, as in #2), but even with the comma removed it is not really right. This is at least partly because it is awkward to combine “my”, which is itself a kind of definer, with another defining clause.

The second sentence is OK.

Sentence 1 could be construed as a defining clause without the comma if you mean ‘my sister’ in the sense of one of my sisters.

“One of my sisters whose flat is in central London is a charity worker.”
Is this sentence OK?
What is the reason I should not use ‘my sister’ in place of ‘one of my sister’?

I do not like either “one of my sisters” or “my sister” modified by a defining clause. This is because both “my” and “one of my” already define “sister”, and to me it is awkward to then redefine it. This is despite the fact that someone may obviously have more than one sister. Similarly, I would not say “My car that is red was stolen” in the event that I have several cars.