Need to rephrase any?

Hi. For my questions, let's say the letters "XXX" and "YYY" represent the name of a certain movie and name of the theme song for that move respectively. I am particularly unsure whether the two underlined parts are correct or not. As to the second underlined part "Mr. John Doe's brother's sister," would it better if it were changed to "the sister of Mr. John Doe's brother," assuming Mr. John Doe has only one sister? Thank you in advance for your help.

(A and B talking)

A: Have you seen the movie XXX?
B: Yes, it was great. The theme song for the movie "YYY" was great, too.
A: Yes, I agree. Oh, I noticed Mr. John Doe's brother's sister was at our seminar yesterday. Did you see her too?