Marilyn has given excellent advice here. "Cancer" is a scary term but there are so many amazing stories of people who have done so well and are cancer-free, so that is what you aim for.
Chemo treatments vary tremendously, and stretching them out over 4 months doesn't indicate the seriousness or extent of the cancer. There are very specialized treatments based on the type of cancer, first of all, and there are different "chemo cocktails" for each type.
Nutrition is key. There are some amazing clinical studies done by oncologists showing great results with certain highly-absorbable supplements that are easily digested (very important during chemo, when nausea is a common problem) and which help with energy (critical during both chemo and radiation). I can send you one if you message me. The more energy and strength the patient has, the better able they are to manage the effects of the treatments as well as keep a near-normal schedule in terms of going to work and participating in life's activities. And all of that helps the mood and creates a positive outlook. For example, a friend of mine has a mother diagnosed with multiple myeloma - and her chemo dose is quite low (the cancer is responding and her "numbers" are down) yet the woman's energy level is better than ever.
You can be there for your mother by going to some appointments with a notepad, and taking notes. It's hard for people in the situation to remember everything that the doctor says.
You can also help by getting educated but be very careful about scaring the pants off yourself! Until we've been personally touched by cancer, the disease is a big scary monster. But individual cases vary so much, and the success rates are incredible. Use the patient and family education resources at the cancer center (and to any on-line resources they suggest), and resist the urge to wildly surf the internet, where you will find everything in the universe, and not all from reliable sources. You can also join a cancer support group - the cancer treatment center may have recommendations. You can educate yourself about the benefits of an alkaline diet and cancer.
And you can help your brother by being positive and keeping his life as normal as possible. He may not freak out and cry - if you all take the attitude that his dad is going to get through this, he may not be so scared. That's really important, not only for a 12 year old as he enters his tween/teen years, but also for your stepfather, who's going to be worrying about his family when he needs to concentrate on his own treatment and recovery.
What to say and do: DO NOT act like this is a death sentence. DO tell your stepdad you are there for him, for your mom and for your brother. DO send him things to do during treatment - puzzle books, light reading (whatever he likes), maybe a play list for an iPod or MP3 player, etc. DO make up a few meals for the freezer so your mom doesn't have to cook if she doesn't feel like it. Go for small portions in case your stepdad can't eat too much at one sitting - that can happen and it's not a bad thing. Drive your brother to activities and keep his life as normal as possible. DO NOT stop having family activities, holidays, birthdays, or your own life. That doesn't help anyone.
It's important to be forward-thinking with a positive attitude. In the "chemo world", 4 months is not a long time. It's a finite treatment plan with a beginning and an end, and that's good!
Try to be positive!