Should I just let it go?

My husbands 13yr old niece, who live 2-3 hours away, was staying with us for a week. She was getting ready to leave one morning and visit some friends. She had made herself tea in the mug that I got on our honeymoon (we have a his & hers set). I said to her: "No matter what you do - do not lose that mug, I got it on our honeymoon." Well she left it in her friends backpack. She and my husband knew she had left it when he picked her up. They did nothing, they did not tell me she left it (this was a couple days ago, and she has gone home since). Yesterday I asked him if it was in his truck. He says "I don't know." After I didn't find it - he comes clean and tells me she left it.

He has just set an example for her - If you do something wrong you cover it up and lie instead of taking responsibility for your actions. I am mad at myself for letting her out the door with it. I would not have been mad if they had come clean and told me about it. Now I am mad because they lied, and he is all pissy at me. He does not think he should apologize to me for anything.

It doesn't matter if it's a mug or a diamond, it's the principal. We are trying for our first child and I don't think I can allow this style of parenting to continue.