Is it permissible for the suitor to marry his fiancée, knowing that the fiancée's mother breastfed the suitor's elder sister, and are they thereby considered siblings through radha'a (suckling)?

Your cousin (son of your paternal aunt) is not a mahram to you, and it is not forbidden for you to marry him simply because your mother nursed his sister. Only the nursed infant himself becomes a child to the woman who provided the milk and her husband. Marriage to him would only be forbidden if there were a reason for mahramiyya (unmarriageability), such as if both of you were nursed by the same woman, or if you were nursed by his mother or his father's wife (milk of the male), or if he was nursed by your mother or your father's wife. The breastfeeding that establishes mahramiyya is five full, satisfying sucklings.