Perhaps not a bright line. But let's take relativism as our foil, where we understand relativism to mean that standards of evaluation are relative to norms, traditions, etc. of societies or groups. (I'm paraphrasing a definition from https://plato.stanford.edu/entries/moral-relativism/ ). If that view happens to be correct, notice that it doesn't leave us without a distinction between morals and manners. Even if relativism is the right meta-ethical view, we still make a distinction within this society (US society for the sake of example) between matters of politeness and matters of moral right and wrong. Close enough for our purpose, we Americans agree that stealing is wrong and not just rude. We also agree that showing up to a wedding in ragged shorts and a T-shirt is rude but not really a moral wrong (though see below). The line between the two cases seems to be something like this: we can imagine, though we might not find it an attractive prospect, that fashions might change and showing up to a wedding "badly dressed" might come to be acceptable. Nothing in the kinds of reasons we appeal to in moral argumentation provides a basis for treating the rudeness of ragged shorts at a wedding as anything deeper than custom. When we offer moral arguments, we appeal to considerations of such things as harm and fairness that we don't treat as matters of fashion. Those sorts of reasons don't get much of a grip in arguments about what to wear to weddings.
The general idea is that even if relativism is the correct metatheory, there is a set of norms, rules and reasons that limn what the group takes to constitute morality. Appeals to those kinds of reasons are what makes an argument about how to behave a moral argument. Other cases, such as whether it's okay to slurp your soup or wear white to a wedding if you're not the bride, don't appeal to those sorts of reasons.
This still doesn't give us a bright line. For example: in the US (and in lot of other places!) it's rude to constantly interrupt people when they talk. Some cases of interrupting don't bump up against the boundary of what we count as moral transgressions. Some people are just socially clueless. But in other cases, constant interruption amounts to not showing respect for the person you're interrupting; it may count as an affront to their dignity. However, we don't just count respect and dignity as matters of manners; they're moral issues too.
So it's doubtful that we'll find bright lines, but we do find not-altogether-blurry ones. In fact (as my earlier answer indicated) this will be so whether or not we're objectivists. The rude can sometimes amount to a wrong.