What is the ruling regarding paternal half-siblings through fosterage?

If the question is about the children of the father from رضاعة (رضاعة referring to the suckling of a child by a woman who is not its birth mother, where the milk is produced due to her husband's intercourse), then they are siblings to the suckling. Marriage to them is prohibited. However, it is permissible to look at them when there is no fear of temptation (ريبة) and in the absence of seclusion (خلوة).