Is a person sinful for not knowing his foster sister or meeting her, due to the prevailing customs and traditions in his country?

Maharim (unmarriageable kin) by suckling have a right to honor and benevolence, but they are not entitled to the same obligatory rights as blood relatives in the specific sense. Therefore, if a brother meets his sister by suckling, he may greet her and maintain ties with her out of honor and benevolence. However, if visiting her causes embarrassment or hardship, he is not obliged to do so and incurs no sin, for they are not among the obligatory kin. This is supported by the actions of the Prophet, peace and blessings be upon him, with his suckling sister, al-Shayma, and his suckling uncle. Kinship is of two types: general (kinship of faith) and specific (kinship of blood), and each has different obligatory rights.