>>96197273 (OP)
Blanks aren't actually all that rare, all things considered.
And the reason the Sisters are all female has not been given.
They exist, as literally admitted by GW, was so the Sabertooth HH card game had a bit of variety, not just nothing but marines and a sausagefest across all the art.
As GW expanded on the HH, the Sisters sat in a weird spot, not existing in 40K, so they had to have been killed off during the HH, specifically the webway war, and GW was content to not actually look at who they were.
As the HH novels continued, the sisters got a little development. Several alternate origins were alluded to: being the private milita of some Navigator family with a bit of a fetish for dommy amazons that got turned into the chamber militant of the psykana.
An independent matriarchial, or all-women, warrior cult that selective bred blanks. Which ties into the totemic naming schemes among the cadres.
To even originally a guard force from a world that had an abnormaly large number of blanks being born, mostly female, and that world is now deeply censored to protect this prime, effectively unlimited source of Blanks.
With recent re-retcons, the Sisters are now a pet project of the custodes from the start of the Great Crusade.
Their all-female nature is unexplained. Their exotic and unconventional hierarchy and organisation is unexplained.
Their totemic naming system is unexplained.
Most of their background is unexplained and everytime GW tries to explain it, it makes even less sense then before.