I'm at a loss where the Bridges element comes into all this! Phyllis was born in 1847 and her sister Janet born 1850. Janet died aged 9, and has Mary Grierson as her mother, when Mary Grierson dies she has her mother as Mary Smith - so not sure how Bridges ends up on multiple separate documents. Unless an error has just been perpetuated.
The person I contacted from Ancestry has got back to me, all his information has come from his wifes side and he has sourced his info from scotlandspeople. He also has pictures of Hugh and Jane and various offspring which have been passed down the generations. Some of Phillis line ends up in the Falklands and Patagonia in the mid to late 1800s - another offshoot ends up marrying the Countess of Suffolk! He's give me some more links to follow, but is no wiser than us regards to Isabella being the sister of Hugh (or indeed their parents)
Bizarrely I have several DNA matches in Argentina (Patagonia), but always assumed they were from my Welsh Ancestors ( one of them who marries Isabella's daugher, Isabella in Liverpool) who went over and set up a Welsh community. They were heavily religious (welsh calvinistic methodists) and I have found some links to say Hugh Smith was involved in presbytarian churches quite heavily in Maxwelltown.
I would love to get as close to 100% on Isabella being the brother of Hugh, there are enough pointers to suggest this is the case - but still no silver bullet yet!