Really I think they'd just be reasonably attractive from their lifestyles compared to most Imperial women who are either poor as shit Hivers or decadent obese nobles. Some Sisters might have potato face but they're all going to be pretty healthy and well groomed and the majority of women look ok if they look after themselves.
Spartan women were famously beautiful in ancient Greece, probably because they had good diets, exercised regularly and took care of themselves whilst most Greek women were locked in the centre of their homes, malnourished as children and exercised once or twice a year dancing at religious festivals.
The Sororitas seems pretty self-contained as an element of the Ecclesiarchy, I can't see some pervy old priest getting the opportunity to pick out girls based on how qt he thinks they are, I'd expect the Sisters to select their own recruits from the Schola Progenium, particularly the Battle Sisters who are going to be trusting their lives to these recruits on the battlefield.
Haven't got anything to back this up, just how it works in my head, given how corrupt the Imperium/Ecclesiarchy is, I can't see high ranking priests having any problem getting hold of nubile young women to molest without having to draw the ire of the Sororitas by interfering with their recruitment. Especially since the SoB do have a bit of a history of executing corrupt clergy if their weaknesses threaten the Emperor/Imperium...