Is it a condition for two children to become siblings through breastfeeding that they be breastfed directly from the wet nurse's breast, or is it permissible to collect the milk and transport it to the child, even if they are in another country?

There is no harm in breastfeeding a nephew or niece to make them a mahram (definition) to the nursing woman and her children. Indeed, Aisha, may Allah be pleased with her, used to instruct the daughters of her brothers and sisters to breastfeed whomever she wished to be able to enter upon her. The breastfeeding that establishes mahramiyyah is not limited to suckling milk directly from the breast; rather, if it is placed in a vessel and the child drinks from it, it is considered valid breastfeeding according to the majority of scholars. It is stipulated that there be five breastfeedings and that the child has not exceeded two years of age.