I am so sorry to hear this news. I lost my brother to cancer several years ago (same sort of situation, only spread out over a longer period of time than 4 weeks), and it pains me to hear of anyone going down the same road. Cancer is a horrible, horrible disease, and I wish you all peace during this tough time.
I agree with the comments of other posters here. Also, maybe keep up the supportive e-mails, though don't over-do it. Maybe a nice, thoughtful, heartfelt email once a week just to say you're thinking of her and her husband. Odds are she already knows that you are there for her and that you are willing to do whatever you can to help, but she is overwhelmed as his wife and (presumably) caregiver. She is probably, hopefully, receiving many such kind offers from neighbors, co-workers, friends, and family who maybe live in her area. In that situation, even the help itself can be overwhelming, when you're getting casseroles and other baked goods thrown at you left and right. It can also be difficult to express gratitude when the emotions (and energy) are pulled in so many directions, as they are when a loved one is dying of cancer. It is good that you already recognize this, as you said that you probably wouldn't respond to e-mails much either if you were in her shoes. Unfortunately, I think this is something a lot of people don't understand, and they can get turned off when they show support that seemingly goes unnoticed.
I don't know how close you are to your brother-in-law, but maybe sending him a personal e-mail or card would boost his spirits, as well.
Financially, if there is any way you can help, then maybe offer to chip in to pay some bills or to defray some everyday cost of living that could simply be distracting at this point in your stepsister and brother-in-law's lives. Maybe talk to your mom and stepdad to see if they can get some "insider info" on certain bills or costs that you could arrange to pay without your stepsister and brother-in-law knowing. I'm sure that would come as a very welcome and very much appreciated surprise to them at this time.
Flowers may or may not be a good way of showing support. You can have flowers sent with a personal message and delivered to their home, but a word of warning on that--when my brother was very ill, some old friends sent flowers to him and his caregiver, meaning well of course, and accompanied by an uplifting "thinking-of-you" sort of note. He was aware of how sick he was at that point, though, and the flowers actually really depressed him because, in his mind, there are only 3 situations in life that call for flowers: (1) surprising your girlfriend or mother (2) Get Well Soon wishes when someone is sick but on the mend, or (3) funerals. He was clearly not "on the mend," and to him the flowers made him imagine his funeral. If your brother-in-law and sister are into flowers, enjoy gardening or whatever, etc., then maybe the flowers would be seen in a different light. I just wanted to throw that out there as a possible caution. I don't know how your brother-in-law's spirits are right now.
Of course, you can never go wrong with the practical stuff, as you mentioned, like gift certificates to take-out places or even to a spa or something for your sister-in-law. If one of them has some special interest, maybe a book for leisure reading--especially for the brother-in-law, who probably has a lot of free time (and maybe not too much energy)--could be a good gift. If they are religious, there are of course many different options down that avenue. An example is in the Catholic Church, where you can have a Mass service celebrated in honor of a loved one and then have some sort of certificate sent to the family to let them know that the church community is keeping them in their prayers and offering the service in their name.
Anyway, this is a lengthy response, but I hope it gives you some ideas. This is a topic that I have personal experience with and feel connected to, so I wanted to really take some time to respond to your question.
Hope this helps. Best of luck, and I am sorry again to hear this news of your brother-in-law's terminal cancer.