Whatever you feel, it doesn't matter in the slightest. Whether she acts like she wants it or not leave her alone, because 1 or both of the following WILL happen:
1. You will warp her. She will be damaged goods for the rest of her life because once she fucked a member of her famiy. Don't put her in the position where she will feel ashamed about herself or her actions. You are meant to know better.
2. Your sister will find out that you are banging her child and either report you (because if you've missed all the other reasons that this is sick and wrong, her age is another one you can add to your list - she's a minor) or just cut you out of her life forever and have to live knowing that her brother took advantage of her child.
1. You will warp her. She will be damaged goods for the rest of her life because once she fucked a member of her famiy. Don't put her in the position where she will feel ashamed about herself or her actions. You are meant to know better.
2. Your sister will find out that you are banging her child and either report you (because if you've missed all the other reasons that this is sick and wrong, her age is another one you can add to your list - she's a minor) or just cut you out of her life forever and have to live knowing that her brother took advantage of her child.
So keep it in your pants, ok?