It's a kind of natural thing you know? She sort of blames Miyuki for the situation they are in, not that she is entirely wrong about it. Paired with a hint of jealousy, and the fact she feels a bit inadequate because of her leg. Neither feeling isn't warranted but it's part of being human.
All that put together with a host of other issues she has, nightmares, PTSD, and she acts as she does. And that probably makes the relationship between Miyuki and Susan the most realistic out of the people involved.
But don't get me wrong here, she might bitch about the things until the end of time, but if the push comes to shove, she will step in front of a bullet that is meant for Miyuki (or anyone else for that matter). Sure, she will bitch about it afterward, but she would do it again in a heartbeat.
Hmmm?