To Tell or Not To Tell The Truth

I was asked today not to tell the truth regarding when a guest who is staying with us arrived. The person does not want their sister to know when they arrived, because they do not want to have to feel obligated to visit her because she is perceived as not being of any fun unlike my significant other. The paradox here is that I have been told in no uncertain terms to always tell the truth except in this situation where I am being told to lie. I am not being asked in either situation. I am being told, not asked. However, as a Christian, my fidelity rests with Christ who is Truth. There is nothing false about Jesus. However, because I cannot say with absolute certainty when our guest arrived as I do not remember, if I were asked when our guest arrived by her sister, I can truly say that I cannot recall.  The moral of the story here is that someone who is bent on the truth should not pick-and-choose when to tell the truth because anyone who identifies as Christian should be of the Truth. In this case, my significant other might be said to be a nominal Christian. However, thank God I am not the Judge.