It's a great theory, one that I consider to be a possibility. The only problem is that Murtagh and Roran are not--repeat, not--related. Eragon and Murtagh share the same mother, not father. Roran was Murtagh's mother's brother's son. They're only related by marriage, not blood.
However, Roran becoming the third Rider is a distinct possibility. I'm starting a new theories thread tomorrow, if you want to read more.