Just talk to her. Tell her anything to get her to shut up. If the family asks him, make sure you and he have your stories straight with one another.
Tell them you were going to take a shower, or that you were changing after work in his room or anything. Make this out as if the little sister misinterpreted what was going on and that her imagination was taking over. Just make sure you and he are telling the same story.
A 7 year old can somewhat understand what you two were doing. I have two of them. You might want to explain it to the parents of the child, because children see and say everything.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.