I am confuddled, you have ruled out the death in 1877 of Mary Ann PEPPER, of Paddington at age 59. (birth year of around 1818) but you have Mary Ann arriving in 1832 on the Red Rover, at age 19 … (birth year of around 1813) Do you have a copy of the official transcription or the actual d.c.? Does it give the details of her marriage, and her children including perhaps Charlotte Mary Anna, whose marriage back in 1859 was registered in Paddington (#1113/1859) … likely she married Amos AVERY, who in the 1870s was a carter of 1 Mary Street, Surry Hills (ie within the Paddington BDM registration district)….
If the only reason you found to rule that out was the age difference, it is only a five year difference, and it is information given by the informant, which if either her son in law or her daughter may reflect what they ‘knew’ rather than her actual year of birth…. An adult person’s exact age was not a critical issue in that era…
Although the online index does NOT display the given names for that Mary Ann PEPPER’s parents, it should give you ‘how long’ in the colony (of NSW) and the name of her spouse, and her maiden name and perhaps even her home townland… So, likely it won’t lead directly to knowing the names and details for Bridget’s origins, it may well be a step closer to your goal, but it may well give you enough info to confirm that Mary Ann was likely to be Bridget’s sister … (NSW BDM ref 2751/1877) ... And it should give you the cemetery for Mary Ann, and oh ... which may well be where Bridget was interred .... and that cemetery's headstones may be transcribed and .... I must not get ahead of myself
JM