Surgery confusion

It was a bit fuzzy but I *think* that the thing they removed was a twin. Sometimes one foetus will absorb another one in the womb, and it will remain inside the person into adulthood, like a tumor. So she probably didn't know she had it, but maybe it had started to compress on her stomach, which made her sick. I'm just guessing here. I didn't really understand what that character's function was at all in this movie. A lot of her personal information seemed to be missing.