« Reply #2 on: Saturday 27 April 13 22:21 BST (UK) »
There is a William Pool (born 1814) recorded in the 1851 census as living at Newbie Barns, Annan with Janet Irving, his mother. The household includes another son for Janet Irving, called Robert Thomson (born 1808). There is also an Irving Carlyle (born 1835), who is said to be also Janet Irving's son and whose father is said to be a William Pool.
Could it be that Robert's father has died (or vanished, for whatever reason) and that Janet has remarried? And could it be that the census enumerator meant that Irving Carlyle is Janet's grandson and the child of her son, William?
As to the later relationships, I've found that it's not unusual for cousins to marry in this period and I have found at least one example in my own family of someone marrying her brother-in-law after her sister died.
Iandj
Iandj
Dumfriesshire: Martin, Lorrain(e), Smart, Muir
Roxburghshire, Selkirkshire: Lorrain, Turnbull, Riddell, Elliot(t), Ker(r), Scott
Lancashire, Cheshire: Johnston, Rutter, Barrow
Ayrshire, Lanarkshire: Jamieson, Glasgow, Thomson, Riddet, Blair
Clackmannan, Fife, Stirling: Simpson, Kirk, Stein, Pryde, Penman, Hempseed, Bauchop
Kincardineshire: Craig, Stewart