This is an upsetting situation isnt it, your sister should have a talk with her Daughter and ask for the details ,and you could do the same with your daughter, curiosity is pretty widespread and considered to be Normal, However the sound of Blood on underwear is what you would both look to address. Maybe you all 4 could have a group discussion and see what comes out, you should be able to read their Faces and see how they behave when confronted ,who is lying and who is not.