I would imagine that there's no blood connection between Nero and Spock, but that the Romulan Empire was greatly weakened after Nemesis and perhaps parts of it forced into submission to the Klingon Empire. Nero is a political prisoner. He's got a group of renegades (maybe former soldiers?). Spock may even be working for his release.
Nero's broken out. Nero's people have a plan to go back and eliminate what they see as the cause of Romulus' descent on the galactic stage. They may not even know what that nexus point is per se, but if they eliminate a given Federation ship at a given time, the timeline will shift to something more to their liking. Nero et al go back. Spock, on the scene working for Nero's release, now must go back to stop him. Maybe he knows where he's going, maybe he doesn't. Somehow, Nero and Spock enter the timestream at the same instant (otherwise, whomever went back second wouldn't exist).
Nero's plan is to eliminate the Federation ship, the Kelvin. It seems to go according to plan; Kelvin destroyed. But the timestream hasn't changed as predicted (perhaps because Kirk, Jr. wasn't killed on the ship). The timeline's different (Kirk grows up entirely without a father and is a bit more of a deliquent because of it), but not the right kind of different for Nero and his cohorts.
So Nero goes back to eliminate Vulcan (maybe Vulcan is ascendent in the new timeline for some reason). Spock's trying to avert further damage without pulling a "Back to the Future" and ending his own existance in a paradox. Spock uses Kirk as his tool and his knowledge of SF protocol and his own psychology of the time to do so.
Nero's plans are f'd up. Nero buys the farm in climactic fashion. But the timeline's still fundumentally changed and now we can have sequals that, similar to Marvel's Ultimate line (or even the Mirror Universe), have the same role players but with different backstories: Re-boot via Re-imagining. No reset button.
Dan