Are the ties of kinship to one's brother's daughters through suckling obligatory, like the ties of kinship to sisters and brothers' daughters by blood relation?

Connecting with and visiting foster-mahrams (unmarriageable kin by fosterage) is not obligatory, unlike blood relatives, for the concept of kinship (rahim) is specific to blood relatives. Sheikh Ibn 'Uthaymeen mentioned that four rulings of consanguinity are established solely by fosterage: marriage, looking (at the opposite sex), seclusion (khalwa), and being unmarriageable (mahramiyyah). It is, however, recommended to connect with foster mothers through visits, greetings, prayers, and offering gifts if possible.