While there's nothing morally wrong with step-siblings becoming romantically involved, it's highly improbably due to reverse imprinting which happens between childhood peers that continue to cohabitate through to puberty. In fact, this is the one reason we don't need to fear a mass of marriages between blood siblings were law to allow it: it just doesn't happen very often.
Regarding the potential of breaking up with a cohabitating step-sibling this isn't much more different than breaking up with an otherwise unrelated cohabitant, especially in this economy, in which finding a new place can be extremely difficult or impossible. In my own case, it's always been a personal code to regard my exes with civility (and despite our histories I can still remember she is someone I once loved). The awkwardness of a recent break-up, as intense as it can be, is transitory, and in eons during which we organized into tribes and bands, the process of forgiveness and re-inclusion was more necessary, so I don't see why, but for the convenience of alternatives, we practice it now.
But for the most part, between live-in siblings, step- or otherwise, it wouldn't happen.
Fun Fact: For most of history in western civilization, it was actually unlawful to marry step-siblings, since they were regarded as siblings-by-blood. On the other hand, marriages between cousins, or (just as often) uncles and nieces (less so aunts and nephews) were quite legal and commonly practiced. Only during the twentieth century did attitudes change.
238U.