What are the legitimate Islamic boundaries regarding my fiancée's interaction with her male cousins who were raised with her in the same house and whom she considers as her brothers?

You were right to criticize your fiancée for her leniency with her male cousins, because being alone with them (khalwa), displaying adornments (zina) in front of them, shaking hands with them, or engaging in talk and jest that opens the door to fitna (temptation/strife) is impermissible. It is recommended to maintain ties with them, like other non-mahram relatives, through greetings, inquiring about them, and kindness, while avoiding anything that causes suspicion. Muthanna said: I asked Abu Abdullah: A man has female relatives who do not stand before him (i.e., do not serve him in an intimate way), so what is due upon him concerning their good treatment? And how often should he visit them? He replied: "Kindness and greetings." And in the hadith: "Maintain ties with your relatives, even if only by greeting them." It is recommended to visit a female cousin and other relatives, provided there is no khalwa or action that causes suspicion or violates Islamic law, and with adherence to the prescribed hijab. Resume advising your fiancée and inform her of the words of the scholars regarding the limits of interaction between non-mahram relatives, bearing in mind that before the marriage contract, a suitor is a stranger to his fiancée, and his conversation with her should be limited to what is necessary.