I'm so sorry. The thing that I can say to you now that might be hard to hear but if you are able to hear it, is to help your father-in-law prepare. Start now. No matter how much a rock he is, he needs help. He needs to get to the lawyer and draw up papers if there aren't any drawn up - quickly before she can't make any sense or before she is unable to sign anything. He needs to deal with power of attorneys and a living will. A DNR is very important so that she doesn't have to suffer needlessly because a DNR wasn't in place.
Help with preparing for the inevitable with the funeral home and cemetary, too, Mama. I can tell you how hard it is to write an obituary from scratch all night long to have to give the funeral home the next morning, because my mom and I had to do it for my dad. It's hard. And it's the first thing that has to be done because of having to be announced in the paper for legal purposes. It is so hard to make choices right after someone dies - if you go take care of it before it happens, the funeral home will handle everything for you as soon as you call them with the news.
You need to tell the kids. It will be hard for them, but don't hide it from them. She will be very different once she starts the chemo. She'll be sick and will look different, and may not know them. You need to prepare them.
Pallative care will be the most important thing to do for her. Find out as much as you can about that. Hospice will be important to you. They will be your godsend.
Seek out your pastor or spiritual guide to help you through this. Encourage your husband to as well, no matter how much of a "rock" he seems to be now.
Start now putting together pictures of her from before, pictures of her with the kids, etc. Pick out a lovely portrait of her for people to remember her by. If you can make a power point presentation of her life, you can share that with family and friends and put together a lovely memory table. This act will also help you say goodbye before she goes.
Sometimes nothing helps but DOING. Start - go piece by piece. No matter how hard. It will help.
Sending you strength and clarity and hugs~
Dawn