Firstly I'm sorry your little girl has fractured her leg. Hope it heals really quickly and she'll be back on her feet in no time at all. Please don't feel bad about not noticing that she had hurt her leg. It is so hard to guess what is wrong with our little ones until they can start talking and tell us what's wrong it really is a guessing game. You did the best you could with the info you had and that was gave her heaps of TLC and settled her.
The issue over working out what to do I don't think is ever going to be solved I'm sorry. Unless the boy will tell his mum/you what happened you will never know. It must be causing friction between you and your sister. I really don't have any idea how to move on becuase I'm a big act first then later and usually just yell (trying not to) so I would have just yelled at him and demanded to know what happened so I probably would have caused a fight. I think it is wrong that your sister is trying to shift the blame onto you. Maybe you could talk to her and ask her to stop. Sorry not much help at all. Hopefully someone else will give you some helpful advice.