Well, according to the scripts Lucy is five years younger than Lois. At least according to The Pilot:

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Lois pops a frozen dinner into the microwave, thenturns to see her sister, in a robe, towel drying her hair, enter from the bedroom. LUCY LANE is five years younger than Lois, a living Barbie whose major goal is to find a living Ken.

"How many times must I tell you? Queens consume nectars and ambrosia, not hot dogs."