Frankly your sister is wrong. It's also silly of her to essentially ignore your silhouette and insist that by measurements you're not. Measurements are very misleading!
For example, by measurements I'm an hourglass (34-25-35), but if you see my silhouette it's clear that I'm much more an inverted triangle with a defined waist, because I actually have a strong shoulder line and very narrow hips but a round butt, so my hip measurement comes from my butt instead! Does that make sense?
And really if you think about it, if you LOOK like an hourglass, what does it matter what the measuring tape says? Aren't you trying to determine how you LOOK, considering that is in fact what you ARE? Besides, there are countless different ways to determine a "true" hourglass; some say the hip-to-waist ratio has to be below 75% some say 50%, some say your waist must be 10" smaller than hips or hips and bust, blah blah blah...they're all just numbers. And believe me, I've read all the different silly ways to figure out what you are mathematically, and nothing has been as helpful as the feedback from a picture on YLF!