Yes. You should always speak up. What did you say to the father in law? I might have said please stop or I’m going to inform your son. Let’s be clear in this day and age if you don’t speak up, he may flip it into something. Make sure you aren’t around your father in law by yourself. For me he’s shown that he’s not to be trusted.
Also be clear that he’s truly hitting on you. Some older men talk slick with their words and doesn’t mean that he’s hitting on you. However if you’re sure that he is and you have spoken to him about it than I would have a conversation with your husband to be sure he understands that you are uncomfortable with it. Good luck.