Catherine was certainly the same person who married James Calder in 1859. By then she and Peter likely had moved to Dunnet, Caithness. In 1851, about a year before Peter was born, the censuses says that Catherine worked on a farm in Hempriggs, Caithness, at the same time James Calder's grandfather and several aunts were there. The aunts were also farm workers, but who worked at which farm isn't stated. The choices were few, so Catherine surely knew the Dunnets. By 1861, James's grandfather John Dunnet and daughters were back in their native Dunnet parish, and the Calders were next door. James, like many people in Caithness, was also a farm laborer. By the time the 1871 census was taken, Peter had four half-siblings. In 1861 and 1871, he was listed as a Calder.
Between 1871 and 1874 the Calders moved to Washington in County Durham, England. Catherine and Peter were working in Washington Chemical Works by 1881 and Peter was a "slagman" at the Washington Iron Works when he married in 1874. The furnace probably made steel, and slagmen took care of the waste products (slag) that were left-over after the smelting process.
James married Jacobina Dunnet, one of his stepfather's first cousins. There was a 15 year age difference, and Jacobina lived in Dunnet. Presumably they met in Dunnet, and may be that the Calders moved to England not long before the marriage. If Peter and Jacobina developed a relationship and married for love, he likely would not have moved. No evidence has been found that they were expecting a child and then married. Whatever the circumstances, the marriage was at her brother George's home at 14 Breadalbane Terrace in Edinburgh. William Dunnet was a witness and was surely Jacobina's brother. A William Dunnet, undoubtedly the same, lived in Kirkwall by the time Peter died, and reported his death to the local authorities.
William Dunnet played an important role in the lives of the Sinclairs. He worked in hotels in Edinburgh and Aberdeen and was butler to Sir James Sinclair of Pentland. The Valuation Rolls for Kirkwall show William as proprietor of the Kirkwall Hotel on Bridge Street in 1876-1877, but not before. Peter and Jacobina likely went with him. Their daughter Barbara was born in West Dunnet in 1877, so they may have lived with her parents after they married. The Kirkwall Hotel occupied what was a large house with ornamental gardens. In 1880 he rented Seatter Farm just east of Kirkwall, undoubtedly to provision the hotel. It was at Seatter that Jacobina and her children Barbara and James were enumerated in 1881. William's obituary says that Jacobina managed the farm. Various other records say Peter was a waiter in the hotel.
Peter died of "general lymphatic disease," probably cancer, which he had for 3 months. His death certificate says he died on Albert Street, but his death notice is surely right in saying Bridge Street, and likely at the hotel. He is buried in the churchyard of St. Magnus Cathedral.