The history of the Sinclair family in Scotland is a subject of intense interest, especially after the publication of The Da Vinci Code. Sadly, this has continued a myth-making process that was started long ago by the Victorians.
For sure, the Sinclairs made their mark on Scotland, but they are not a Highland clan with a prehistory stretching back to early times. The first Sinclair was supposed to have arrived in Scotland from England around 1067, a consequence of the uneasy rivalry between the Scottish king Malcolm and William, the Norman conqueror of England. This was the story told by van Bassan and repeated by Father Hay in his genealogy of the Roslin St. Clairs about 1700. However, it is just as likely that, according to a contemporary chronicler, the St. Clairs accompanied king William "the Lion" on his return to Scotland in 1174 after imprisonment in Normandy.
Alan de Sinclair and Henry de Sinclair both received land from William de Morville prior to his death in 1196. William was Constable of Scotland and a strong argument has been made by historians that the appearance of St. Clairs in Scotland had much to do with the de Morville family, who knew of the St. Clair family in England and Normandy.
Alan was a witness to a charter by Hugh (II) de Morville, William's grandfather, and previously Constable of Scotland, and the land he and Henry received later became part of Herdmanston. What has yet to be shown is whether another St. Clair received land at Roslin at the same time. If that was the case, it would increase the likelihood that the founders of Herdmanston and Roslin were from the same family. Roslin, after all, is only a short distance from Herdmanston.
There were several early charters witnessed by men with the title of de Roslin, but the first mention of a St. Clair in connection with Roslin was when Henry de Roslin passed his lands to William de Sinclair. Father Hay could not be sure whether this charter was issued during the reign of king Alexander II or III, so the date might have been either 1245 or 1280. But William de St. Clair was Sheriff of Edinburghshire (Edinburgh, Linglithgow and Haddington) in 1264 and Sir William Sinclair of Roslin was appointed Sheriff of Edinburgh for life in 1271. This predates Roslin being passed to William during Alexander III's reign, so the first Sinclair to hold Roslin must have been the William who received it from Henry de Roslin in 1245. Henry de Roslin might then have been the same Henry de St. Clair who held Herdmanston, and arranged to pass Roslin to William, one of his sons, and Herdmanston to another son.