I'm not an expert, but I believe she's allowed to go through the US line with you because she's your family and you're travelling together.
The best advice I've seen on this question, though, is to look for the immigration person helping to direct traffic prior to the lines and ask them what you're supposed to do. That way if you get any grief from the immigration officer you have somebody to point to to say, "They told us to do it this way."