Brennan's brother (https://bones.fandom.com/wiki/Russ_Brennan) disappeared after the third, for example, and while he did get mentioned on occasion, he never physically appeared on-screen again. The same goes for Booth's brother, Jared (https://bones.fandom.com/wiki/Jared_Booth), to the point where they just decided to kill him off. And Finn Abernathy (https://bones.fandom.com/wiki/Finn_Abernathy), one of the interns, just disappeared without a trace or real mention of him again