Lol, when I was 13 years old, my cousin (same age as me) was on the phone with a BOY. I was sitting on the floor, flipping through a book while she talked.
Convo:
"No, I can't, my cousin's here."
*pause while he talks*
"Jessica."
*pause while he talks again*
"She looks like me, only fatter."
It was like being slapped in the face. I will NEVER forget that moment. And ever since then, one of my primary motivations for family gatherings is to look better than her. When I was about 18, she gained a pile of weight, and I'd lost. She was being particularly snooty and rude about something, and I subtly worked that scenario into the conversation somehow... "remember when you said this--" type thing. Her older brother, bless him, snickered and said loudly, "Gee, look who's fatter now."
It was horrible. It was. Sure. But I enjoyed every moment of it, and I won't feel guilty about it, lol. ...at least your competition isn't malicious!
My point is, there are always going to be people you're competing against, and sometimes you can't help but want to rub their nose in it, because it builds you up a bit. It's only human. My advice? USE it, honey. You've got an opportunity here. Feel sad, and frustrated, and crushed that she's doing a bit better at the moment? Or throw your efforts into kicking her butt.
10 lbs? Honey, you put some effort behind it, you could be down that in a MONTH.
I have ABSOLUTE faith in you, I know you can do it! Once the two of you are on-par, you can kick-*** together.