Well, it's clearly stated that Brahne wasn't always the way she is. She adopted Garnet and was apparently very kind and generous, hence why Garnet notices the difference in behaviour since Kuja came along.
I don't know if I go for her being mind-controlled though, I think she really was seduced. She was also in a fragile state because her husband had just recently died, and that's when Kuja started showing up. When she was dying she apologized for the things she had done, rather than saying that she was under an external influence or that she couldn't stop herself.