Mmm, I still disagree. "I don't remember" isn't backing anyone. Why would she think saying she didn't remember would keep her in Joffrey's good graces? IMO, if that was really what she was going for, she would have said Arya was the liar. I think she was just scared and thought "I don't remember" was the safest thing to say. She couldn't possibly have known Cersei would demand Lady's death, and Mycah was killed before Arya and Sansa were brought before Robert and Cersei, so Sansa had nothing to do with that.