Mulder's sister Samantha may also be Spender's sister. In "Redux II," CSM
introduces Mulder to a grown-up Samantha whom he claims is his daughter. And Tena
Mulder, Samantha's mother, did apparently have an affair with CSM.
Samantha may know of Spender's existence, and even that he's her brother. If she was raised by CSM, he may have told her about his other child, Jeffrey. It's doubtful that Spender knows anything about her, though.