I was a transcriptionist/receptionist for a Hemotology/Oncology group for about 3 years. My mother is still employed there and its really hard to see some of the patients. Most of them are so sweet, their families come in on occasion for support and the staff is also very supportive of them. If your mother is so blessed to find an oncologist nearby I hope the staff is the same. Often times when you visit the oncologist you will recieve a packet of goodies, information and sometimes sample medications to help offset the cost. In this packet you should be able to find out some about support groups, websites and books which can help to better explain what is going on. If you are unable to find anything in your information ask the staff, and they should be more than happy to help.
Just be there for her and your family. Check in on her to make sure she is taking her meds and that shes going to her appointments. Sometimes patients get depressed, and give up hope and quit taking the very meds that are keeping them alive. Your siblings and your family will appreciate the little things even moreso now that they are in a time of need. Just like Cyote said dont stress yourself out over this, that wont help. Keep your spirits up and try to answer any questions your siblings may have. If you cant answer them ask the doctor and then explain it to them.
Also if there is ever a Relay For Life in your area take your mother and talk to the survivors. She can participate in the "Survivors Walk" and make a new group of friends who can also help her through this. I dont know when they begin but the one in my area is around March. This event is sponsored by the American Cancer Society and you can probably find out more information on their website listed below.
here are a few websites for colon cancer and colon cancer support groups:
www.ccalliance.org
http://www.cancer.org/
http://www.sharedexperience.org/
Im including a few links for cancer centers through Universities Ive had personal experiences with. They may or may not be close to you but think about checking into a similar University Cancer Center if there is a large University near you. If your mother has insurance they may cover it and if your mother is willing she may be eligible for clinical trials(this is of course a decision for her to make) and they may have support groups on site for your mother and your family(because this is a big leap for your family as well) that will benefit both of you and ease the stress.
http://www.mdanderson.org/
http://www.mayo.edu/
http://www.ufscc.ufl.edu/
I hope these sites and the further diagnosis of your mother help you and your family. You are definately in my thoughts and prayers considering this has come at the Holidays.
God Bless.