Daughterinlaw,
There is no reason to feel silly. We all need a helping hand from time to time.
The best advice I can give you at this point is just to be there. Stay involved with your father inlaw. Accompany him to his doctors appointments. Ask questions, listen, write notes, be an advocate. Learn about his treatment options. Please feel free to ask any question that comes to mind. There are some fabulous people on this site who are more than capable and readily available to answer any questions you may have.
You did not mention in your post whether your father in law lives with you, or close to you. Will you have help in dealing with his care after treatments? Just a few things that came to mind that will make a huge difference in how you and your husband and children deal with this.
Believe me when I tell you that this is not a death sentence. I will not sugar coat it either, it will be a tough fight. It will test you every step of the way. But there are many of us who have walked in his shoes, so let him know that it can be done.
Good luck with everything.
John