Is a son attributed to his biological father, even though his mother did not breastfeed him naturally, and her husband has a sister, so should the child greet his mother's husband's sister, or should the mother reveal her secret?

If the questioner's situation is similar to that of the sister for whom a fatwa (definition) was issued, attributing the fetus to her husband, then her child's relationship to his father is one of lineage, not fosterage. Consequently, her husband's sister would be the child's paternal aunt by blood, and she is among his unmarriageable kin (mahram (definition)) and relatives (arham). One must conceal one's sins and not declare them openly. Caution must be exercised against a woman being lenient in her interactions with non-mahram men. She should not allow anyone to enter upon her or be alone with her unless there is someone present who negates the unlawful privacy according to Islamic law. She must adhere to modesty, covering, and hijab (definition) when going out or in the presence of non-mahram men.