One way of interpreting the letter is that Emanuel Slifer thought that Peter Miller and Peter Gaver were both brothers-in-law of John Slifer. Another way of interpreting the letter is that two un-named brothers-in-law of John Slifer came to what is now the Burkittsville area, in addition to Peter Miller and Peter Gaver.
Peter Miller was married to Peter Gaver's daughters Elizabeth and Rebecca, and I think that John Slifer was married to Peter Gaver's daughter Asenath Gaver. That doesn't make them brothers-in-law, at least not the way I understand brother-in-law relationships.
