I believe that leaseholds were heritable property themselves, which might explain the service of heirs for your Wyllie family. I have a document relating to a family in Argyll that says that one John Campbell took out a 9-year lease on a farm just before he died. A younger son took over the lease on his father's death, but some years later John's eldest son and heir claimed right to the set.
I will order the appropriate FHL film. Hopefully the document will at least give me an idea why Janet's son and daughter living in Scotland are not named as co-heirs along with their siblings in Canada.
I do no think that Janet would have owned any heritable property in Scotland, especially since she had left Scotland in 1821. The main reason why I thought that the service of heirs had to do with the fifty pounds is that Janet's brother Matthew specifically says that, if Janet dies before she can receive it, the money should go to only the children living in North America. This makes sense, since Matthew gave 80 pounds to her children in Scotland.
Captain Matthew Anderson did not have any children, and the main purpose of his Disposition & Settlement is to give his heritable property (including the lands of Broadlie and his houses) to his younger and only surviving brother John Anderson and John's heirs. Matthew describes the annuities and legacies as burdens and conditions to John or his heirs owning his heritable property:
"But declaring that these presents [Matthew's land & buildings] are granted and shall be accepted of by the said John Anderson and his foresaids under the burdens Conditions and declarations following"
"I do hereby declare the whole annuities and Legacies before written to be real burdens on and affecting my said heritable subjects particularly above disponed and I appoint this declaration to be inserted in the Infeftments and other Investitures to follow hereon so long as the same are undischarged under the pain of nullity"
From what I have read, heritable property is normally restricted to immovable property such as land and buildings, but that other property could become heritable by desingation or by connection to heritable property. Perhaps the service of heirs was required because paying the legacies are conditions to John and his heirs possessing Broadlie and are thus connected to heritable property.
That said, I still do not see why John Anderson, brother and executor to Matthew Anderson, would not have had the power and ability to divide the 50 pounds among Janet's children in Canada himself without the need of a service of heirs.
The only other people in my family who had a service of heirs were those inheriting the lands of Broadlie.
Stefan Ramonat