You may have local options that are less pricey. There is a local company where people can go to create several meals in a couple of hours. It's set up assembly-line style. Here is a link to one in our area:
http://www.letsdish.com
There may be one with an option to create individual meals.
I think this would be considerably less expensive than having meals shipped in. The three sisters could take turns once a week going in and assembling a week's worth of meals for your Dad.
Edit: I started looking at some of the details on the website, and I see that they have ready-mades. That might be an even better option for you. I think at this particular place they change the menu every week.
Disclaimer: I haven't actually done it, but I know this place has been in business for about 5 years now so there must be lots of other people that use it!