You are correct, the common mediaeval usage was that the senior lady of the household (usually the lord's wife, or if he was unmarried his mother or older sister, or the task might fall to an older daughter if her mother died) kept the keys of the household, so she could keep track of stored supplies, household tools, household linen, the spice chest and all the domestic stuff. It was her job to supervise the repair, maintenance and supply of everything that the household used in its daily or seasonal running, keep track of what needed resupply or replacement, manage the servants, make sure the younger women knew how to sew well and that they acquired all the lady skills, do or supervise the brewing, baking and stillroom work, not to mention the spinning, weaving and making of clothes. Also to keep household records and do it all with style and economy. A busy life!
The keys were often large and quite fancy, because they were a mark of the lady's rank as well as her domestic position, and were worn on a long cord attached either directly to her belt, or more often to a decorated pouch containing her personal stuff such as scissors, sewing needle in its own little case, comb, eating spoon and earspoon.
