Is Mei adopted?

Mei and Satsuki don't physically look much like sisters, and Satsuki is designed to look like her two parents, whereas Mei doesn't really look like either. Also, Mei has some attachment issues that Satsuki does not have (ex: copying everything Satsuki says to be "more like" her sister - yes, i know this is also just something that little sisters do - or the running away sequence). I've tried looking up more about this and haven't found anything - any help out there?