My Daughter in Law is lying?

My Son's common law wife is not telling the truth about something i told her should i confront her or just let it go. I was at there house looking after her Daughter because she was sick. I fixed a large picture that they got from my house and put it over the fireplace to see how it looked. I wondered if they would like to see how it looked before taking it down as it was not fixed proper to the wall and was worried about it falling. So I mentioned it to my Son what I did, But my Son does not always listen and seems to forget things quick so I mentioned it once to my Daughter in Law. Then again before putting on my boots and leaving I told her again that the picture was over the fireplace but needs to be taken down. About one hour later my Son calls me and says thanks alot the picture hit my Daughterinlaws little girl 11 years old on the head why did I leave it there. I told him I told you and the wife. Long story short she lied and said I did not tell her when I did tell her twice. Then she also told my Husband the same thing when he was at there house. I really care for this girl but I am starting to piece together things that she says that seem to come out different as time goes on. Should I just let this go and not say anything. I feel strong about the truth and I do not care for lies. But as a Mother of Sons I do understand that one needs to pick their battles wisely as Sons favor on the wife's sides and I do understand this and do not want to lose my Sons in my life. However no matter what my Sons have done something wrong I may be upset about and I may disappointed even as Men now they Tell me the truth and I do not always like what they may have done but always appreciate that they respect my enough no to lie to me. That being said, Should I just let this go and say Nothing? Not to cause problems in the family.