Katherine made us what we are, and then later decided to kill us because apparently we'd 'squandered her gift' by trying to ... well, by succeeding in killing each other. Which was also her fault. We thought her dead for a number of years, until she returned, completely insane and tried to kill us.
Johanna could be a direct descendant, they look nearly identical. Once she discovered this, Johanna took it to mind that she could succeed where Katherine failed and bring us together, one big happy blood-sucking family. The only flaw with this plan is that she's trying to do it much the same way Katherine did, and she seems to think it a misunderstanding that we think she's crazy.