I've worked with a lot of friends, neighbors and colleagues who have cancer, and the needs vary. Sometimes the greatest offers of help are actually a burden, because the patient and spouse have to do all the organizing/coordinating.
We've done a couple of things. First of all, it's important to know that the effects of chemo and radiation can worsen over time. Let's face it - the stuff is poison. So even if someone does well after the first few weeks, that can change over time. While your sister going out there for a week with her adult daughter and a young child (not sure of the age) seems like a good idea, it might not be in April! If they are staying with your parents, that may put a burden on your mother and father that's just not helpful.
I've done a lot of work helping chemo and radiation patients keep their energy level up - radiation saps the strength and can also cause skin burns, and chemo of course causes a lot of nausea and vomiting. So eating can be very problematic and getting enough nutrition to absorb is difficult. Making meals is a good thing, BUT right now you have no idea what your mother's appetite will be like and what she will feel like eating. If the chemo changes her taste buds (which often happens) and she doesn't really want all the meals people have put together, it can be a waste of time, effort and money. With all the clinical data available on increasing energy and increasing absorption of nutrients, and with the new advent of the first discovered super-food that inhibits cancer cell metastasis, it's often easier and far more effective to use something easy they can manage at home. I have just been to several long training sessions in cellular epigenetics, and it's been phenomenal seeing the slides of the breast cancer cells that can no longer divide (and so they die) just from exposure to a natural plant peptide. It's changing the way we look at cancer treatment.
Sometimes the food ordering from a grocery store just becomes one more task the family doesn't want to deal with. They have enough planning and scheduling with appointments, and taking each day a little at a time until they see how Mom feels.
There is a great website you can use/manage from afar that helps coordinate all of the needs - not just food, but also things like rides to appointments, someone to sit with Mom to keep her company and give Dad a break, tasks around the house they don't have time or energy for (errands, shopping, yard work, you name it). It's called Lotsa Helping Hands, it's free, and it just requires a few designated administrators (maybe you, your sister, and 1-2 people in their area - maybe a neighbor, friend, someone from church or a club they are in - but no more than 4 or it's not manageable. You can put in the dates & tasks needed, let people sign up, you can put in meal preferences and things Mom wants to avoid if she already doesn't like something or if a food aversion develops, delivery instructions (so Mom and Dad can leave a cooler on the porch so they don't have to open the door/entertain), and a whole host of other things.
What happens a lot is that people say to the patient, "What can I do?" - that leaves the patient with a JOB, to coordinate all the well-meaning friends, at a time when the patient has no focus! So LHH lets the patient communicate with one or two people who then communicate with others. You can set up a site for Mom, send that to whomever wants to help, and keep it off Mom's plate. If things change (such as an appointment schedule or a no-longer-appetizing food), you change it once on the website. Someone signing up to bring a meal can see a) food choices b) available dates/delivery instructions and c) what others are bringing on the days immediately before or after. The site also sends out reminders to people telling them they are supposed to do something on April 2nd or May 3rd.
I'd resist the urge to do too much right now because, frankly, none of you know what she's going to want or need! I know that's so frustrating.
On the hats/scarves - a lot of chemo centers have displays of headgear that patients can try on and purchase for a small fee (like $5 - many are donated by knitters, for example). It might be good for your mother to see what's available and what looks flattering on her. She might not be ready for that right now - it depends on what the likelihood is that she will lose her hair, and when in the process that might occur. Some people like to get a jump on that, others like to wait until the last possible moment because they just can't handle it right now.
Wishing you all good luck.