A study of lost causes. By Sir Bruce Gordon Seton. Edinburgh : Lindsay and Macleod, 1939-1941. 2 volumes. Reproduced from typewriting. House of Seton, Vol. 2, page 97-98. https://digital.nls.uk/95447891
There is a charter which appears to have been overlooked in this connection.
On 16th April 1346 Sir William Douglas of Liddesdale bound himself to make certain payments to "Sir"Alexander de Seton, Knight, Lord of that ilk", (i.e. to the Sir Alexander Seton quartus, who was defender of Berwick), in consideration of the marriage of "Alexander de Seton, son of umquhile (deceased) "Sir John de Seton, heir of the said Alexander" with Margaret, daughter of the deceased Sir William de Ruthven.
This charter obviously shows that, in April 1346, both the sons of old Sir Alexander, viz Sir Alexander quintus and Sir John, were dead; also that the heir male to the estate was then Alexander Seton, sextus ,son of Sir John.
Moreover, the fact of Alexander sextus being styled heir, also shows that Alexander quintus had left no male issue. But, as stated above, Alexander sextus himself died, or was killed in the year of the charter, when he was married. This relationship may be regarded as the only one that can be accepted.
The only possible doubt is whether Margaret, who undoubtedly succeeded, was the daughter of Sir Alexander V "the son", or of his brother "Sir John", or possibly of the other brother William who was drowned at Berwick.
It is assumed that the succession as shown above is correct, in spite of any statements by other authorities .