Be proud that you Sister is so enchanted with your work. I have four sisters. Two of us are very very close. The other two just , well, anyways, cherish what you have. Make a drawing of her! You will have her forever frozen in time, not in a photograph, but in a drawing that shows what you see of her, which can be quite different than a picture, just make sure you're happy with her at the time! She will never, ever, forget you for that. Looking at what your doing, I bet you do portraits just fine.