Love ya Tess! I know what you're going through, because I was right beside it with my own family member for months dying of cancer. Cancer is an evil, unfair, painful thing. But know that ANGELS are there waiting for your FIL. Be the best comfort that you can be right now to your DH, your MIL and yourself. You can't see it now, they can't see it now, but he will forever be with you WHENEVER you choose to listen. Whenever you choose to STOP and talk to him. He will become your guardian angel, always watching out for you. Hang in their girl - but relish in the blessings that he has brought into your lives.
Hugs to you and YOUR family. I'm sorry you have to go through this.