Supermarkets--except at train stations and airports--are generally closed on Sundays in Germany. There are no special restrictions on alcohol sales on Sundays that I am aware of, but you will have trouble finding an open grocery store outside of a train station or airport.

The store opening hours rules are set by each of the 16 Länder. Hessen, like a number of other Länder, allows stores to be open 24 hours per day every day but Sunday. In practice, however, few stores take advantage of that. Many larger supermarkets are, however, now open until 10pm or so Monday-Saturday (a marked improvement over the older rules/practices).

There are also a number of locally-determined Sundays when shops are open (often coinciding with a festival, etc.). Supermarkets generally don't participate, but the markets in larger department stores (often there is a large food section in the basement level) usually do.