The Limits of Interaction with Siblings through Breastfeeding

Question: What are the limits of interaction with siblings through breastfeeding—males and females? For example, is it permissible for my brother through breastfeeding to travel with his sister through breastfeeding, or to stay with her in the same house, or to kiss her as he would kiss his sister by blood? I ask for clarification on this, and what are the exceptions that are not permissible between siblings through breastfeeding?

Answer: Breastfeeding is like blood kinship, as the Prophet (peace and blessings be upon him) said: . So his sister through breastfeeding is like his sister by blood. There is no harm in being alone with her, no harm in traveling with her, no harm in shaking her hand, and no harm in kissing her. However, the kiss should not be on the mouth—that is what is preferable—rather, it should be on the head or the cheek, as al-Ṣiddiq used to kiss his daughter Aisha on her cheek.
The conclusion is that it is the same as blood kinship; due to the saying of the Prophet (peace and blessings be upon him): . However, breastfeeding is not like blood kinship in maintaining family ties, for kinship is specific to close relatives. This applies only to the prohibition of marriage and being alone, not to financial maintenance or maintaining family ties; due to his saying (peace and blessings be upon him): .