A total of 41 letters from the comedy legend, who was born in Ulverston, to Nellie Bushby between 1947 and 1965, go under the hammer on 16 September.
Newcastle auctioneers Anderson & Garland expect them to sell for up to Â£12,000.
Last year, one letter from the star to the wife of a friend, sold for Â£1,400.
The letters are owned by theatre buffs Rodney and Margaret Hardcastle, from York, who have built up a large collection of Laurel and Hardy memorabilia over the past 40 years.
Mr Hardcastle, a former chair of York Theatre Royal, said the couple were selling the collection because of a decision to downsize to a smaller home.
Mrs Hardcastle said: "We've always valued the letters, not in a financial sense, but through being able to have them.
"We bought them at Sotheby's in the late 1990s. We couldn't go to the auction because of work, so we thought we'd put a bid in and we didn't think we'd get them."
Stan Laurel, who was born in 1890, wrote to his cousin from his home in California, while filming in France and on tour in the UK.
They recall several spells of ill health and his thoughts following the death of Oliver Hardy. One of the letters was written days after his screen partner's death in August 1957.
The collection also includes theatre programmes, play bills, and a copy of Mr Laurel's birth certificate.
Anderson & Garland auctioneer, Fred Wyrley-Birch said: "The letters give a fantastic insight into the personal life of a very public man."
A collection of letters written by Cumbrian-born Stan Laurel to his cousin are expected to fetch thousands of pounds when they come up for auction.