in the begining i thought roger was luffy's granpa and the goverment threatened roger that they would kill his family if he didn't surrender
but know that we know garp is luffy's granpa,even if roger is garp's brother,the WG would never heart luffy since was too young to do anything then and luffy was destined to be a marine by garp
so my theory on roger being relative to roger hasn't "dropped" yet but since they couldn't blackmail him and i doubt someone could touch his nakama i think roger was betrayed by the inside,a one piece judas
i think someone from the old admirals was in his crew and worked with the WG and set up roger in exchange for a high position in the marines (cuz i doubt he scichibukai existed then and maybe that inspired their creation)