I think Mae did go to the light side. They made a point of dressing Mae in white and Osha in black for a reason. It's also not to my mind a neutral act to make the massive sacrifice of forgetting the sister you have loved and longed for your entire life in order for her to be safe. Even if Osha was dark side/Sith, it was still a sacrifice to save another.
As for Osha being sad, you still have your other emotions when you go to the dark side. The dark ones are just dominant. For all that Osha was sad, she didn't hesitate to let Mae make that sacrifice for her, or try to figure out ANY other way to accomplish safety without Mae losing her memory.
I still think that if one is light, the other must be dark and vice versa.