Okay, she's been to the doctor and nothing is wrong with her but what did the doctor say about the fact that she cries non-stop? I can understand babies have colic but she's a toddler now, so it can't be that! Is she especially sensitive to teething? They're basically getting teeth from six months until two years, but even then you get a break every once in a while! Does she ever play? Does she make eye contact with you? Does she interact with her twin and older sister?

It sounds really puzzling. Obviously your other two children are happy. I know you said the doctor checked her but if absolutely nothing makes her happy then it truly sounds to me like she's in pain that she can't communicate to you. Maybe try a neurologist?

Good luck, please let me know.