Aristotle gives sincerity a narrow, technical sense that the modern English word has since worn smooth. In Book Four of the Nicomachean Ethics he is working through a list of character virtues, each one a mean state between an excess and a deficiency of some disposition, courage between rashness and cowardice, generosity between wastefulness and stinginess, and when he reaches the virtue concerning how a person represents their own qualities and achievements he calls it truthfulness, aletheia, a word this atlas renders as sincerity to keep it distinct from the separate virtue of truth telling about facts in general.
The excess is boastfulness, claiming for oneself more credit, skill or honor than one actually has. The deficiency is a kind of false modesty, what Aristotle calls irony or self-deprecation, understating one's own real qualities, which he treats as a fault in its own right and not merely a safe distance from boasting. The sincere person occupies neither position. They neither inflate nor deflate the report they give of themselves, and Aristotle is explicit that this is a matter of habitual disposition rather than occasional honesty: a single true statement about oneself does not make a person sincere any more than a single act of restraint makes them temperate.
This sits outside the Christian architecture that organizes most of the other entries in this section. It is not one of the three theological virtues, not one of the four cardinal virtues, and not one of the seven virtues later Christian writers paired against the seven capital sins. It belongs to a longer, less systematized catalogue that the Nicomachean Ethics works through virtue by virtue, several of which, wit, friendliness, proper ambition, have no equivalent named virtue in the Christian scheme at all.