When my mom and dad separated back in 2001, I was devastated. It hit me like a truck and I just had a VERY hard time dealing with it...and I was in my 20's!
I lived with my dad after they separated/divorced and we got along great. Then, in 2005-2006, my mom started coming back around. Eventually, they were re-married in 2006. I was conflicted and mostly not happy about it. I tried to put on the glad-you-are-back-together face, but inside...I was just not happy about it. There are a million reasons why that I'd rather not share here.
Perhaps that is what Stevie is feeling too. She's no longer the only girl, she had gotten used to that and she had come to grips that Christine was gone for good. Now, she's back, Lindsey is super excited about working with her and I think Stevie thought she would be able to work more on solo stuff and other interests. Now...that has changed.
I dunno...just a theory.