ANDY'S ANSWER: Let's face it, puberty gives people plenty of material to make fun of. Cracking voices, hair popping out all over, growth spurts, etc... all make for great jokes. I remember my best friend's brother teasing me about the brillo pads sprouting under my armpits. It really made me self-conscious until I realized that my friend's brother sounded just like Marge Simpson whenever he said something. Once I had something to tease him about, my brillo pads didn't seem so funny to him anymore. We made a deal to lay off of each other's puberty weirdness.
Puberty is just one of those things that we all have to go through to get to adulthood. I'm sure you've never let an opportunity go by to tease your sister when you've had the chance. She's just doing what sisters do. The more upset you get, the more she'll enjoy it. Might as well see the humour of it yourself and laugh with everyone else.