You've no reason to be sorry for complaining and asking for help. (-:
Seeking out comfort is what you should do if you're in a situation you're uncomfortable with and unknowing of. More often than not, complaints and concerns towards oneself are seeds being planted that have the possibility of helping you grow and bloom into the person you want and desire to be.
Anyway, you sound quite young. If the middle sibling is only 16, I can't imagine you to be very old... Now your body shape is what you are most worried about. At your age, while your body is still developing, diets and starvation will do absolutely nothing but hinder your growth.
If you allow your body to grow to its fullest height, and eat well, you will obtain the curves you crave. It's a fact, 70% of what your body looks like is dependant on what you eat, whilst only 10% is determined by exercising. (The other 10% factors into other random fields)
This doesn't mean you have to diet. Just maintain a balanced nutrition, with 5 meals per day. (Portions are usually too largely made due to the traditional 3 meals per day. It's a scientific fact that 5 small-medium sized meals are much better on the body and mind.)
Furthermore, because you're young and still growing, you will more than likely follow your middle sister's development pattern and your 'chubby' body well mature into curves.
You mention you've a small waist. DO YOU KNOW HOW LUCKY YOU ARE? That's a great asset! It proves your body has the ability to be very voluptuous. (;
You don't need to money to become pretty, (though I can't lie, it does help) all you need is the determination to become who you want to be.
Remember, the first 17ish years of a girl's appearance DRAMATICALLY changes due to over-active and raging hormones.
During the 8th grade, I was notorious for being the class nerd. Glasses, shaggy clothing, messy and boring hair that framed a freckle invested face. No boy ever liked me. I was small and scrawny.
But now I'm 17, going into my senior year of high school and I've changed to the point of no resemblance to my younger self. I now model for multiple different clothing lines, and have had both boys and men ask me out on dates.
My self esteem used to be dangerously low, and because of this my older sister would use it to her advantage. Please don't let your siblings have power over you. Look into a mirror and tell yourself you've so much potential to be beautiful, just like your sisters. And you've got the heart to match it. (-: