This has been theorized in pretty much every Geass thread since Rolo appeared. Example:
>Lelouch joins the SAR along with the Black Knights, who are very skeptical about the idea. Lelouch reveals to Suzaku that Rolo is working with the Black Knights, and hopes that Suzaku will take Rolo away to the mainland now that he's reunited with his sister. However, Rolo, who doesn't want his position as brother usurped by Nunnally, decides to take matters into his own hands and try to kill her. Suzaku stops him, but believes that Lelouch is behind it, and decides to finish off the Black Knights, who were expecting treachery since the Euphie incident, swear to destroy Britannia.
>Oh yea, Suaku's "Live!" command? It kills a Geassed Anya.
Nunnally probably isn't going to die, though, seeing as how she's billed as a "Main Character" by the writers. Suzaku's going to stop Rolo before he kills her. It would be pretty hilarious if she killed Rolo, though.