It sounds quite suspicious to me that your nephew was involved in some way, 1pj. I agree with the others, and I wouldn't be subtle - I would be simly telling the child's mother that you know he lied, that he has been caught lying and being rough before, that this time there has been a serious consequence, and that unless she is willing to constantly supervise his behaviour while he is there, that he is not welcome. That is a drastic move, but for a seven year old who is so undisciplined that he cannot even manage to play gently with a 10 month old, I think drastic action needs to be taken.

We had something similar happen to us when DD2 was 9 months old. We accidentally dropped her in our back garden, and she sustained a spiral fracture to her femur. I can assure you there were no twisting forces involved! Children's bones spiral fracture because they are more pliable and therefore tend to bend and buckle before they break completely. That said, one possible scenario comnes to mind in your case - you mention that your nephew said she "rolled and hit her head" - if she rolled while her leg was trapped, possibly under him if he was lying on her or being rough with her, then there would be some twisting force involved.

Children's bones heal fast. DD2 is three now and has absolutely no deficit after her injury.