First, if she is eating healthy and is active, have you taken her to the doctor to find out why she is overweight? Does she sneak high calorie foods or are you providing other meals that are high in calories that she is overeating? If the weight gain is recent, it might have something to do with your recent marriage. If it's emotional, I would recommend letting her speak to a counselor about what's going on with her that's making her overeat. She's only 10 so if her eating is not emotionally motivated or she doesn't have some other underlying health issue that's causing her to gain weight, my best advice is to not make a big deal about it.
I was an obese child and I think I ate more every time my mom would mention anything about my weight. I think if my mom would have had healthy foods in the house, and didn't constantly nag me about my weight, things might have turned out differently. Instead, we ate very high fat meals, had high calorie snacks available all the time and my mother would sabotage me all the time by giving me treats as rewards. I am trying very hard not to do that to my children.
My daughter is slightly overweight. I think she eats more than the average child her age, so we've learned to keep healthy snacks like fruits. lean meats, whole grain breads, and vegetables available for those times she says she is "starving" and can't wait for dinner. We let her participate in soccer, at her request, as a way to get additional exercise outside of school P.E. and just general kid's playing and running around. Also, whenever she asks to get on the treadmill, we always say sure. She thinks it's fun and we're happy she's getting some extra exercise. We never ever tell her she needs to be more active or exercise more or anything like that.
I also try very hard not to mention anything about her weight. When she wants snacks, I usually let her have one "unhealthy" thing, like a bowl of cereal or a peanut butter and jelly sandwich (always on whole wheat), but she knows if she wants anything else, it has to be something healthy. We pretty much let her have an unlimited amount of fruits and veggies. And our deal is that I don't care if she doesn't eat anything else on her plate at dinner time, but I expect her to eat all of her vegetables. I do that so that she knows she has some control and at the same time, I know she's getting the vitamins she needs to be healthy.
Bottom line, if you are providing her healthy food choices and opportunities for activity and exercise without making her weight an issue, she should begin to see some progress. Again, don't make it an issue and don't think it will change overnight. My step-son, who's now 21, was obese when he came to live with us at the age of 7. I did the same for him that I'm doing for my daughter, and by the time he was in 8th grade, he had lost so much weight he was no longer big enough to play a lineman in football like he had since his dad began coaching him in the peewee league several years prior. It was very gradual, but now he loves fruit and will usually choose that for a snack over chips or some other unhealthy option. He's very fit and lean and doesn't seem to have any food issues at all now.
My approach would be just to give her love, time and healthy lifestyle options and if she doesn't have any other underlying physical or emotional issues, it should all work out ok.