What is the ruling on a young man seeing his aunt (mother's sister) who is immodestly dressed, and what are the consequences of lustful arousal resulting from that?

Love for the sake of Allah is a Muslim's love for his brother due to his righteousness and obedience to Allah, not for kinship or benefit. If your maternal aunt appears before you in tight or transparent clothing, this is a reprehensible act (munkar), for a woman's 'awrah (intimate parts) in front of her mahrams (unmarriageable kin) is her entire body except for the extremities, and it is impermissible to look at anything other than that. You must admonish her kindly; if she does not respond, then boycott her if the boycott will benefit her.