I think she had, at the very least, guilty knowledge of the murder. I don't think she liked Meredith very much and was largely apathetic about her death. The story she initially gave about covering her ears seems quite natural to me, even if she backed off that claim later. I think she helped with the cleanup and the coverup, even if she wasn't in the room at the time.
The thing that stands out most to be wasn't the cartwheels or the giggling or whatever, but the fact that she was able to walk in that open door and take a shower in a bathroom with a not insignificant amount of blood. If she wasn't there, didn't know, then you can't chalk that up to shock. The stories they gave about that night were full of inconsistencies.
I don't know if she held Meredith down or used the knife or goaded the boys into it. I do think she has enough guilt to make the screams of injustice ring largely silent on my ears. She hasn't done much to help herself. She could have largely mitigated her sentence by telling the whole truth instead of wholesale denials of any guilt. I can believe she's innocent of first degree murder, but there's no way you can rationally detach her from the crime completely, IMO.