Question That girl who is wearing a red hat is Tom's sister Jane.

Dear native English speakers,
do these sentences sound natural?
(a1) That girl wearing a red hat is Tom's sister Jane.
(a2) That girl wearing the red hat is Tom's sister Jane.
(b1) That girl wearing a red hat is Tom's sister Jane.
(b2) That girl wearing the red hat is Tom's sister Jane.

Someone says,
They're natural if Tom has more than one sister.
For instance: "That girl wearing a red hat is Tom's sister Jane. The one in a blue hat is his sister Jill."
If Tom only has one sister, then you need a comma: "Tom's sister, Jane."

What do you think about this remark?

Hirashin

Did you miss a comma from b1 and b2? Either way, all the sentences sound natural to me.

Generally a comma is used after a non-defining clause, so there is a case for putting a comma after "sister" when there is only one sister. I don't think I would use one if I was writing though, however many sisters Tom has, because I think the sentence looks better without the comma. If I wanted to make the distinction between Tom's only sister and one of Tom's sisters, I'd probably add in some extra words (Tom's younger sister Jane)

In fact I do have two sisters, Jenny and Jemima. If I'm out with Jenny and I see a friend who hasn't met her, I'd simply say "This is my sister Jenny" and would probably not feel any need to mention my other sister unless it was relevant.