The Sisters may be found anywhere from street corners, where they offer to tell ones future for a few pennies, to those who appear to be working for the service of a lord. It is said that they go where they will, when they will they work for who they choose, not for who chooses them.
A secret MLM (pyramid) company that offers eternal life, but in fact it transfers the life forces of its members to those position "above". If you get high enough, (meaning you get enough new members) you start to enjoy the benefits. Maybe a willing sacrifice is much more effective for the transfer. But no one says, that a suddenly unwilling member, threatening to tell the authorities, may not loose it involuntarily... thus start people to vanish, and aged corpses are deposited somewhere safe.