Is the wife of an unknown-lineage brother, whom neither my mother nor any of my female relatives breastfed, considered a stranger to me, and what should I do when visiting them?

The uncle's wife is not a mahram (definition) to a man; she is a stranger to him, even if the uncle is a blood relative or a foster relative. This applies even more so if he is not such a relative, and he is a stranger to the women of his family. There is no harm in visiting the uncle at his home, with the important condition that one does not seclude oneself with his wife, and does not see her without the legal hijab (definition) or shake her hand.