I think once your sister has her full diagnosis with staging and gets her schedule of chemo and/or radiation, along with the timing of the chemo, you will have a better idea of how you can help. If they would like to have other children in the future, then it is important that she has an OB fertility consult. Time may be of the essence, but one can ask to be put on Lupron which will induce menopause and shut down the reproductive system and help protect it.
Anyone coming to help must have had the flu shot. Also masks should be available in the house should anyone develop any sort of a cold as she will immunocompromised. Keep hand sanitizer around and insist on washing hands.
1- She will definitely need help the week she gets chemo done so is it possible to involve your parent(s) and her husband's parent(s) to just plan to take turns on coming out to help for a few days after chemo. She may actually need someone there with her and certainly to care for her baby maybe for a few days as she just may not be up to be left alone with an infant AND Dad probably will need to work.
2- While you are out there now can you develop a list of her/their friends and what they have offered to do. Then catalog for her, her husband and you make a copy of to perhaps get in place for her:
a- Those who might be able to come and stay with her the week after chemo, especially if family coverage is limited.
b- Those that they could just drop the baby off at any time of day if there was an emergency and needing to get to the doctor or ER. Or also to watch the baby during regular trips to doctor.
c- Those who would be able to contribute a meal for her.
3- On meals (and even possibly infant care) if they belong to a church, this might be a great outlet to see if a chain could be set up on perhaps 3 meals to cover two nights the week of chemo. If with a close group, someone might even volunteer to set up an online calendar. Also when you or family members are out there, try and cook/freeze a few "simple" and yet nutritious meals. It will help her husband if he can learn to work with a calendar and fill in meals for a week/shop.
4- If they could cut back in some areas and afford to have someone come in and do the house cleaning for the period of treatment that would be a really big help. AND/or on help with the infant when it would be best for them.
5- Money might be tight, but if there is a way to find an older "Mother's Helper" to be with her in the late afternoons when baby might be fussy and just need to be engaged with her right there, but not up to it. I would look for 13/14 year old. Again maybe asking through your church for a responsible younger teen or area list.serve.
6- Small things that will consolidate or cut down on errands will be important like as you suggested seeing what can just be purchased in bulk and delivered. Is there a store one can place an order online and Dad or someone just drive by and pick up. When a family member is out there to help, covering the evenings with the baby and seeing if one can do a big food shopping/errands run.
7- You will have to see how the chemo impacts your sister before you really know what help is most needed. Chemo is cumulative so she may feel much better in the beginning. And make sure she and DH know to let her doctor know of any side effects as they should be controllable now.
Now to end on a positive note, our oldest daughter was diagnosed about nine years ago in October with Stage 3 Hodgkins lymphoma and in short order had chemo for 9 months - I believe a treatment and then 3 weeks off and then radiation. She was single/dating at the time and shortly after chemo ended got engaged, married. They waited the advised two years on pregnancy and on the first try had twins who are now healthy almost six years old. Back then her treatment had to start in the hospital, and an OB consult was rushed, but fortunately put on the Lupron. Contact the local LLS as she can be asked to be matched with "a buddy" as our daughter was and it was good for her back then to talk to a girl who had been worse than she and still had had a baby.
It may be important for your sister to do research on proposed courses of treatment as some are more caustic than others. One needs to look at the rate of secondary cancers from the treatment such as breast cancer AND also the rate of infertility. Hodgkins is a cancer of mostly the young and one of the most treatable. Actually, I just remembered it has been 9 years as we plan to do the Light the Night Walk next year to mark 10 years. It will always be important to do follow up and to have a PCP who listens for even any slight changes in one's health. In the early years, I know she went to the cancer center for bloodwork/x-rays and then six months out to her PCP/OBG as this way she is getting checked to some degree every six months. She has been healthy, is the main breadwinner and Mom of two active kids. It is a journey, but there is great hope for a full recovery with this disease.