My question: Why has Islam forbidden a man from marrying his sister through rada'ah (suckling)?

It is obligatory to believe that every divine ruling has profound wisdom, whether we perceive it or not, and our lack of perception does not negate its existence. As for the prohibition of marriage between siblings by suckling, the reason is that they are in the position of full siblings; their flesh and bones grew from the same milk. The connection between them is innate, preventing any desire for intimacy, and the prohibition of marriage protects this innate disposition and prevents the replacement of sibling affection with lust.