What is the ruling regarding the children of the neighbor who suckled my child five full, satisfying times, in relation to my wife, my daughters, my mother, and my sister?

If your neighbor breastfeeds your child five full feedings before he completes two years of age, she becomes his mother through suckling (rada'a), and he becomes a brother to her daughters only. As for your neighbor's sons, they are not mahrams to your wife, your daughters, your mother, and your sister, if your wife did not breastfeed them.