Marriage Between Sunni and Shia: A Heartfelt Question

Salam everyone, I'm a Sunni brother and I genuinely developed feelings for a sister who is Shia. We cared for each other deeply and were considering marriage. I recently asked her to speak with her parents, thinking mine would eventually agree, and assumed hers would too. Unfortunately, her family refused and are pressuring her to marry a Shia brother, even saying she'd be dead to them if she didn't. Is it really such a major sin to marry outside your sect? I've always practiced Islam and believe what's happening to us is unjust and haram. I feel incredibly hurt and broken, my heart literally aches for her. I wonder if those in such situations will be held accountable before Allah, and will I ever see justice? Even if the relationship wasn't permissible, the pain is real and I'm struggling to forgive her parents for this. We all will return to Allah one day, so why do we allow such discrimination among ourselves?