the advantage of my sister or the advantage of my sister's

How it might be phrased would depend a great deal on what someone was trying to say, Adverb, and their personal feelings towards that beauty, whether they felt it was a positive thing or a negative thing.

Here are some possible examples;

One advantage that my sister has ...

One advantage my sister enjoys is ...

The advantage that my sister enjoys/uses/lords over/

If you insist of wording it the way you asked, I would go with "advantage of my sister."