Dear Pretty Screwed Up,
I don't know if I would label you as pretty screwed up, maybe pretty screwed over.
You Are Her "Plan B"
You seem to have a good grasp of the situation, you are occupying a position in Sara's life commonly known as "Plan B".
She knows if Eric doesn't work out, she has your arms to run to. And you seem to be content with that situation, (excepting the fact that it has yet to occur).
She's Using You
She's using you. Plain and simple. In my mind, friends don't play one another like that, so I wouldn't even count her among mine. If you choose to, it's your choice.
Keep in mind that you describe her as sweet and kind, but she sounds to me like a selfish little girl. She's looking out for number one why don't you?