Here's what Elspeth MacRorie's entry in Codex II says:
She was born on the 16th day of March in the year of our Lord 840, being the eldest daughter of Ballard II MacRorie Earl of Culdi and Ardis Lady Drummond, and elder sister of Camber Earl of Culdi. She early expressed an interest in the church and entered the Convent of Saint Hilda's in the year 858, being elected Abbess of that establishment on the 27th day of June in the year 889. She was killed by the forces of the Regents on the year 918, aged 77 years; her body was later recovered and buried with full honours in the Abbey cemetery
It is interesting that we don't really learn much about her from the books. Some abbeys were likely more cloistered than others, but an abbess would be expected to keep fairly well informed about current events, and could probably have family contact if she wanted to. Based on the early vocation she expressed, I doubt that her family opposed her decision (she did have 2 other brothers besides Camber, and a younger sister Aislinn) so it's not like the family had to have her marry and continue the bloodline. I'd agree that the lack of contact is a bit mysterious, but we don't see a whole lot of time before the year of hiding. That would have made contact difficult, and it wasn't too long afterwards that Camber 'died'. It might have been hard to explain contact between her and "Bishop Cullen". Since she entered the abbey 20 years before Joram was born, they probably didn't have much, if any contact, so he might not have been likely to keep in touch.
In checking CH for info on the Statutes of Ramos, I came across a line that said that Camber had never been especially close to his sister Aislinn and only met her grandson Adrian a couple times. (Camber did spend a good part of his youth being educated for an anticipated church career until his brothers died.) If his relationship with his 'worldly' sister was distant, it wouldn't be unexpected for him to be even less close with his 'religious' sister. Rhys and Evaine's decision to foster their eldest son with Adrian could have been an attempt to close the distance.
Based on her entry, the regents knew by her surname that she had to be deryni. She was old enough that some might not have know she was Camber's sister, but they'd at least have known she was a relative. With her age, gender, and vocation, I honestly don't know if she would have tried to resist, or just accepted it, the way the monks at Saint Neot's did.
No way to know if the Abbey was predominatly human or Deryni, or an equal mix. The small numbers of Deryni in general would suggest they might not have been the majority there. And even a Deryni abbess wouldn't guarantee a portal there. Just become the major cathedrals had them, and the Michaeline, Gabrillite, and Varnarite establishments did, doesn't, I think, also mean that ALL religious institutions had portals created in their premises.
After typing all this, I checked the Codex to see if Saint Hilda's was listed. This is what I found.
This convent is located a few miles from the ancient city of Grecotha, having been erected in the year 844 by Mother Fusca, found of the Souers des Sagesse et Culture. Elspeth Lady MacRorie was elected Abbess here in the year 889, but was killed by the Regents' soldiers on the 14th day of January in the year 918 during the anti-Deryni persecutions, her Abbey also being destroyed.