I think this issue should be discussed between you, your wife, and her. What city is your step daughter from?
When she left was she already attending the university in Russia?
Give her some wheels and some friends and life in American will be grand too.
The issue is really beyond her green card status. She needs to decide where her life is. It is unrealistic to think she will return to the US after finishing school in Russia. If she does it is a lot of lost time to get your education and occupation skills up to par compared to someone who went to school and work here in the US for 4 to 5 years.
If anything I think it is just home sickness and strange customs here in the US that is pushing her back home. Perhaps she feels uncomfortable in her new family setting? I understand it is hard for you because Russian is not your primary language and it is hard to get your points across.
In the end it is a choice she is going to have to make and not go change her mind later. Has she considered life apart from her mother?
Since she is only 17 I would have enrolled her into High School rather than a university, even if she has already graduated. High School is a great way for a teen to get a feel of American culture and customs and acquire lots of friends.
But really, there is not much advice I can really give. Good luck, this issue might turn out to be a lot harder to solve then getting the visa in the first place