Alright, firstly, she is 12. At 12 I looked awful, really awful. A lot worst than the other girls my age, and now at 17 I get more complements then any of my other friends. So, give the girl time.
As for weight, again I was pretty fat at 12, now I'm relatively slim. Just sort of, a normal weight. Just encourage her to drink plenty of water, eat some fruit and veggies and get a little (not loads) just a little exercise.
She's 12, remember, a kid basically, let her enjoy childhood as long as she can. After all your a long time an adult. if she was getting as many complements as her sister, I would be concerned, she's a child.
Also, remember there is no one pretty, what some people think is beautiful, others think is just plain ugly. Pretty is socially defined and no human is truly ugly so long as they live a healthy life style.
As for surgery, she shouldn't get any, if SHE, wants to at 18, then that is HER choice. But some people will judge her for being fake, others will think it makes her look good and some people wouldn't care either way. The thing is, she'll never please everybody, so let her make her own path.
One day somebody will love her for her, something that as her mother, you should be the first to do. Looks are not everything, one day we get old and our beauty crumbles, then all we have left is our heart, make sure that is her most wonderful feature, as it never really leaves. I know this isn't what you necessarily want to hear, but it is true. Old age strips us of our pretty mask, and we must make sure what is underneath is kind, loving and beautiful.
Have a peaceful day and remember to love your daughter, if you don't she may not be able to do the most important thing of all, love herself. And there is nothing more sexy than a lady who loves herself. So when she's 18, make sure she feels happy and confident with who she is, and everyone will love her! :) Those who don't, are not worth her or your time.