I may be reading impaired today but I didn't see enough (any) evidence that the brother and sister are "that kind of close". I mean define footsy. Is it one kicked the other or is it a long drawn out romantic table massage? And then...how did she see it? She said: "If I were to bring this up, he'd be upset and think I don't like his sister, which he's accused me of in the past."

Not sure anyone can conclude one way or the other on limited information but I was glad to see at least both potentials (writer's jealousy or sista-love) were mentioned in the response.