Lynata, you take Sandy Mitchell to task for not doing enough research on the Adepta Sororitas but then admit to making the same mistake, not having actually read Cain's Last Stand to see if your assumptions are accurate.
I have read the book, your assumptions are not accurate. Sister Julien is many things, but a "drunken ****" is not one of them. Sister Julien is shown to play a card game yes, using the Emperor's Tarot no less, because that game acts as a cover so the most experienced Imperial military officers on the planet can begin planning said planet's defense against a possible Chaos incursion during the 13th Black Crusade. Sister Julien has unique insights that her decades of combat experience give her, on par with the contributions from the navy officer, the ex-storm trooper colonel, and the commissar. I'm not sure how her attendence in the game, including its social drinking (she is never shown to get drunk or even slightly tispy) which in reality is a command conference, turns her in to a "loose girl you might get to play strip poker with".
I think it speaks highly of Sister Julien that not only has she hidden her skills at the game from the rest of her colleagues, but she is also better at a game that requires strategy and thought than even the book's protagonist. Should a Sister not be good at strategy and forethought? If it eases your outrage any, she even says she will take her winnings and donate them to the Ministorum the next day. I fail to see how that conflicts with any of the fluff regarding the Adeptas at all.
As for your accusation that Sister Julien sleeps around with any one with an Y chromosone, there is only one instance where it is implied that she is having a physical relationship with one of the other teachers at the Schola. And I stress implied, it is not shown 'on screen' as it where. Amberley Vail's footnote on the subject does indeed state that the Adeptas do not require an oath of celibacy, but she also continues on to state that the duties and the life of a Sister does not leave much if any time for them to take advantage of that fact. So your concerns are explicitly addressed by the book in question, and again would not appear to violate the fluff.
So Sister Julien is a competant tactician, a strong willed woman, and loyal to the Emperor to the core while taking her duties very seriously. She is is neither a drunkard, nor a ****, nor some sort of floozy who disregards her duty to have fun. She does actually have some fun in her life, which is what seems to upset you so much. Why is denying millions of fictional future women any degree of happiness, pleasure, and enjoyment of any of life's simple things so important to you? The Adepta Sororitas are made up of human women, the female half of the population of the Imperium of Man, yet you seem to want to deny them their humanity. I think Sister Julien is a far more interesting and compelling character than the other Sisters we see in the Cain series because she is quite clearly human, while at the same time unique. I don't understand why that disqualifies her as a good example of the Adepta Sororitas.
As for the Schola itself being gender-segregated, except for 'religious occasions', the Schola does indeed appear quite gender segregated for the purposes of the Adepta and the Ministorum. Only in Cain's cadre of commissariat cadets does there appear to be any mixing of genders, with the inclusion of one female cadet in his charge. Over the course of the series, and Last Stand is book 6, Mitchell's gone out of his way to explain that guard regiments are almost universally gender-segregated, with male regiments far outnumbering female. The Valhallan 597th being an exception because it was the cobbled together remains of an all male and an all female regiment. The all female regiments function best with female commissars. Cain is the only commissar on the planet, the only one at the school, it makes no sense to run a completely separate cirriuculum for one female cadet.
The rest of the schola shows no signs of being gender integrated. There are no co-ed dorms. The commissar cadets do get to occasionally watch the sororitas trainees run through thier paces on the obstacle course, but that is only because Cain likes to teach outside, atop one of the towers of the schola's defensive wall. The schola students do eat communally, which makes sense due to the Schola's small size and the small population of the planet. I live near a private Islamic school, once in high school my band had a concert there because their auditorium had superior accoustics to my own (public) school. We got to walk through the school on our way to the auditorium, it was gender segregated as more conservative Islamic teaching requires, but even there in a real world analogue to what the Schola would appear to be, there would have been plenty of opportunities for the male and female students to see one another and exchange glances, brief hellos, and the like. Unless every Schola in the Imperium, every where and always, has the male school on one side of the planet and the female school on the other, I don't see how the Schola on Perlia violates fluff either.
I personally really enjoyed the Cain series, and yes I fully understand that it is a lighter, softer 40k than just about anything else out there. Given the still quite heavy amounts of GrimDark in the series, I don't see that as a failing. One of the things Mitchell is good at is consistency, both to the established canon and across his books. I would without hesitation recommend the Cain series to anyone, it offers up another viewpoint on life in the Imperium, especially life in the Imerpial Guard, and on what being used by an Inquisitor is like. Try it, with an open mind, you might like it.