Please explain to my sister what would happen if Edward could read Bella's mind?

I actually think that they wouldn't be together if he could.

The very first thing that attracted Edward to Bella (I think) was his inability to read her mind. He noticed that all of the other students' thoughts in the cafeteria he could clearly hear, except for hers. This is what had Edward begin to be drawn towards Bella.

True, Bella's scent and her character itself also spiked Edward's curiosity.

But I have the feeling that her mind being so private and immune to vampires' gifts contributed immensely to Edward and Bella falling in love.

well i think that if edward would have been able to read bella's mind it would not have drawn him towards her the whole reason edward was drawn towards her in the begaining was because of her blood and the fact that he could not read her mind. I don't think that bella would have like all her pirvate moments to be shared with edward cause she worries about jake alot and she dosn't want him to ever know how much the change to a human to vampire felt and when he left her. I think it would really ruin there realtionship.

(If Edward was able to notice her regardless of her "quiet thoughts")
The Good: Edward and Bella would be able to FULLY understand each other and Edward might not've left Bella in New Moon.
The Bad: The story would lose most of it's plot and mystery, making it less interesting and addicting- Twilight would probably be wrapped up in 1 book instead of 4-5 (if wewe count the Bree Tanner book).

cuz he was a vampire

well, i think he would be even zaidi protective. because she is always saying in her m ind about how she thinks things and is glad he can't hear her because he wouldn't let her do things thinking it was "dangerous". like going and seeing jacob in eclipse and how she was oanicing about their honeymoon in the bathroom that night. There relationship wouldn't be the same. and they possibly wouldn't be together. That's the main reason he talked to her the first day.