Awww poor bubba girl I personally would be pushing the point with your nephew, he has lied to your face (& his mother's) by saying she rolled & hit her head when clearly it wasn't her head that was injured. At this point his mother isn't going to discpline him, so maybe broach it from the direction of "you aren't going to be in trouble, we just need to tell the doctor what happened so we can get DD better." As for your sister, that makes me so cross. SHE should have pushed the point with her DS & she most definitely shouldn't be trying to place the blame on you.

it's such a horrible situation & I know I would be feeling the same way.