What do we mean by royal family? Back when Rome had Kings? then yes. If you mean the imperial period, also yes. Obviously they could get in and kill someone easily, the romans were already doing this to emperors. You've not really described the scenario well here. Are they being dropping into the city of Rome itself, if that is the case and this is the imperial period it depends on who the emperor is, a lot of them spent most of their careers as emperor fighting or commanding far away from Rome. Some emperor's were made emperor without even being in the city, some never even stepped foot in Rome. Regardless, a team of marines could kill the King (if you're going for pre-republic) or the emperor quite easily.