Hah. But yes, when talking about conspiracies in general and the Lahmians in specific it's important to remember that, almost by definition, they're discreet.
Two people can keep a secret if one of them is dead, as the old saying goes. The more people who are in on your secret conspiracy of vampire matriarchs pulling strings from behind the scenes (or secret conspiracy of Tzeentchan cultists or Skaven agents or Ranaldian puppetmasters or...) the less secure that conspiracy is. There are absolutely going to be members of the Sisterhood of Shallya out there who are in thrall to the Lahmians, either directly or indirectly, and a few might even quite literally be vampires themselves... but the vast majority of the Cult will have no idea they exist and would honestly be rather appalled if they found out.
The overwhelming majority of Shallyan priestesses are exactly what they seem to be; pious, hard-working woman fanatically dedicated to caring for the sick, providing for the downtrodden and alleviating the pain of those who suffer. They bring supplies to plague-ridden villages, oversee food distribution in towns hit by famine, and help any expectant mother who asks through the trials of pregnancy and birth. More importantly, they do this indiscriminately - Shallyans are not only absolute pacifists who stay carefully out of all kinds of politics, they're one of very few factions in the Old World that will comfort and support mutants as much as anyone else, if not always openly. It's hard to understate just how ridiculous, absurdly popular they are across all levels of society as a result.
That popularity and acceptance is what makes them a perfect vehicle for others to use as a manner of infiltration and subversion, but it only exists because such things are so rare as to be unthinkable.
...also Shallya is an actual Deity who probably gets very annoyed at people trying to undermine and subvert her faithful, and while she herself is an absolute pacifist, her father loves her very much and is also the literal God of Death.
