I was very lucky that when my parents got divorced and re-married, I had a fantastic experience and even when my half-sister was born when I was 15, my mum ensured that we were not forgotten in the excitement of the new baby.
Being 16 is a really tough time in anyone's life, but especially for a young girl. She's finding out who she is in the world and sometimes is unable to appropriately express emotions - I know I couldn't! Especially when it feels like no one is listening to you sometimes you turn to posts online, just like you have in this forum.
I think you might need to remember that you have entered this child's life, bought your own family with you and turned her world upside down, even if that's not what you have intended. Of course there is going to be jealousy and tough times, but you're the adult in this situation and you need to guide your step daughter by not only speaking to her (preferably with her biological parent as well) but also by acting as an example. I know I used to get so upset with my step mum when she'd tell me to do things but wouldn't do them herself.
If this situation is part of a pattern of behaviors, maybe you need to sit down with the biological parent and really figure out what your next step is. How you can help your step daughter feel like she is part of the family and that all the kids are just as important as each other? I know for me, having dedicated alone time with my biological parents away from my step parents really helped with this.