Violet Baker, who died in April 2013 at 85, inherited the money from her husband, Raymond, who died in 2009, the Daily Mirror reported. Her bequest, 769,000 pounds (about $1.3 million), is the largest ever to a British political party.
Elsie Clark, Raymond's sister, told the Mirror Violet Baker never showed much interest in politics. She said she and her siblings inherited their father's strong Labor sympathies.
"To do what she's done with it -- she's done that for spite," Clark said. "All that money was left to her and she didn't want any of us to get our hands on it. She was a wicked woman."
Clark, 88, said her brother, a career soldier, lived carefully, saving much of what he earned: "It makes you wonder if it's worth saving your money and you should enjoy yourself instead."
Violet Baker lived modestly in the Birmingham area, and a neighbor who took care of her in her last years said he had no idea she had a lot of money.
A Conservative spokesman said Baker made her wishes clear.