Quite frankly, and please understand that I am saying this as gently as I can; it seems like both you and your stepdaughter need to go to counseling.
When I was her age I also gained about 100 pounds. I was living with my mom and step dad. My father was out of the picture. I identified with adults because until my mom remarried she treated and talked to me as an adult. I was not fat at the time but a little chunky and didn't always get along with other children so I spent a lot of time with adults. She also counted on me a lot to help her get through after Dad left. We had some really tough times then.
When she remarried suddenly I was just supposed to become a child again. I can't tell you how much that hurt. She would tell me "Go outside and play with the other kids" or "Get your butt off the sofa and do something." or "This doesn't concern you. Go away." First abandoned by dad then mom wants to shove me into the background again as soon as she gets another man. I am worthy of being a friend and help only when there is no one better around. I know now that wasn't true but it was how I felt.
To top it all off the step parent thought I was fat even before I started to gain weight and had a lot of contempt for me because of it. It seemed like none of the remaining adults in my life liked me or wanted me around.
He never called me a hippo to my face (as I assume you have not either) but I knew he was disgusted by me. Kids aren't stupid and even though he never called names I could see it in his face and hear it in his tone. I knew that the way I looked was more important to him than who I was. They rode me about my weight constantly but continued to bring junk food into the house. They told me if I would "just try" and stop "shoving food in my face" I would loose weight. They also told me that I just needed a little will power and they were not going to make everyone else give up cookies because I couldn't "control myself".
What I heard was if I would loose weight then they would accept me and I would be worth loving. I heard that who I was didn't really matter, but how I looked was very important. The fact that I was taking college level courses didn't matter. The fact that I would graduate early didn't matter. I was on a state championship chess team. Didn't matter. A fat person can't be a good or worthwhile person.
This is about the time I developed my eating disorder. Starving myself for as long as I could and then binging, eating thousands of calories of food I sneaked or stole or had blown my allowance on and the weight started piling on. I would eat until I vomitted from too much food and then eat some more. I gained 100lbs between 6th and 8th grade. By my freshman year I weighted 200lbs. By the time I graduated I was close to 250.
I had no friends because I was fat. I was constantly bullied. Children are terrible. If you daughter is 100 pounds overweight then she is being bullied. Do you want to know how bad it can be? Imagine the worst treatment you can and then know that it's even worse than that. They used to throw rocks at me and old food. When I was walking down the halls in high school guys used to shout "I love me some fat p***Y, more to f**K!" and that wasn't the worst. I would tell teachers and they would tell me I was lying or worse, that maybe I should loose some weight. Some of the teachers even laughed when they saw the kids "teasing" me. I told my mom and she told me to "ignore it and they'll stop." and would then try and get me to go on a diet. In otherwords, "Quit complaining. You deserve it because you are fat."
To make a long story short I tried to kill myself when I was sixteen. My mom wanted to know "how I could do this to her."
I don't generally share this part of my life with others. Its very painful, but I felt it was important in this case. Why? Well, because I too was very concerned about the words you used to describe your daughter. Even if you never voice it to her, trust me she is aware of the anger you feel. Even if you never say it but only think of her as a future hippo, she knows it.
In hindsight I can say you really need to get her to counseling. Between the weight gain, her needy behavior with her father and her avoidence of people her own age there is a problem and she needs someone "safe" to talk to about it. If she has gained 100 pounds in a year I suspect she has a budding binge eating disorder. Obviously I don't know your family dynamic or her school situation but I can tell you that the depression and self loathing the accompanies this behavior is immense, even without outside pressures.
The immediate advise I would give to both you and your husband to not put pressure on her to diet. Get the junk food out of the house and work on serving healthy food. If she expresses the want to go on a diet, encourage her to work on eating healthier and getting regular exercise instead. You might want to google "health at every size" and that will give you some good ideas. When dealing with any kind of eating disorder dieting is the last thing you want them attempting.
Finally, and again please know I'm saying this a gently as I can, you may want to consider counseling as well. It is a difficult position for you and very frustrating. You need someone to talk to as well, without having to worry that anyone is going to think you are "tearing her down" to her father or putting him in the position of having to feel like he must choose who to defend. I would also say, and mind you, this is only an impression from a very brief paragraph, it seems to me like you are competing with your daughter for her father's attention. You may want to watch this dynamic. You are a parent not the other woman
Don't let her manipulate you into that role.
If she says you are a bad person then just let it ride. It sucks but that's the way it is. Any time she changes the subject from what she has done to you, she is attacking you to get the focus off her behavior. Just say "that may be true but we aren't talking about me right now, we are talking about "insert behavior here" and get right back on track.
Finally, this is something I always wished my stepfather would do. If you don't already, quit referring to her as your step-daughter and just call her your daughter. I think it would show her that you do consider her your child in every sense.
As with all advice feel free to take it or leave it. It is the internet and I don't know you or your family and I can only guess according to your post. Blended families are always tough and we all know the Brady Bunch is a fairy tale. I know you are doing your best. Hang in there!
Christine