Nancy Spumoni is an unseen character mentioned in "Arnold's Christmas". It is unknown if she is an actual person or simply the name of a fashion line. If Nancy is an actual person, she likely has some kind of association with Dino Spumoni, since they have the same last name.
In "Dino Checks Out", Dino's revealed to have been married five times and had no biological children of his own (though some of his ex-wives had kids from previous relationships), but none of his ex-wives (or former stepchildren) are named "Nancy." So, assuming (once again) that Nancy Spumoni's a real person but isn't Dino's daughter, a (former) stepdaughter or an ex-wife, she could be a niece, a cousin or possibly even his sister.
Nancy Spumoni's most famous product is a pair of glitzy snow boots, which Helga wanted as a Christmas present. Her "Nancy Spumoni Signature Snowboots" are an obvious reference to the 1966 hit "These Boots Are Made for Walkin'" by Nancy Sinatra, daughter of Frank Sinatra, whom Dino Spumoni is based on.