Complex sentence: John's elder brother, Charles I, Count of Nevers and Count of Rethel, had no legitimate children, and so on his death in 1464 his titles passed to John.
Split into simpler sentences: John's elder brother was also his predecessor in his titles, Charles I, the Count of Nevers and Count of Rethel (1414--64). Upon Charles' death in 1464, he having no legitimate children, his titles passed to John.