My sister made a huge mistake in my opinion

She lost her best friend who fell in love with her and developed feelings for him as well. She will not date him because they share cousins and my ex aunt in law married my uncle long ago and they divorced and there is a family link. They are not related by blood at all. It’s something she can’t get over. I know it is her choice and I have to accept her decision. But I think it’s such a ludicrous reason to not date someone and their evolving relationship dynamic made it difficult for them to only remain best friends. They met at church actually as adults and apparently interacted with him once as a child. He was very good to her and I liked him a lot for her. I thought they would make a great match and they were so compatible. I know she is not ready for relationships yet again after getting raped last year and she has lost another friend from him developing romantic feelings for her but she didn’t feel the same. Her last friend still tried being flirty with her and it made her uncomfortable some months ago. She pushes people away. It’s so heartbreaking I cried. She cried over this last friend because she does really like him and it is a great loss for her. They would still be on speaking terms just creating some distance to let the feelings fade. She could be missing out on the love of her life. It really hit home for me because I lost the love of my life when I broke up with him after I pushed him away after realizing I had a psychotic break with reality. I regret ruining the relationship every day and wish we could have been a real life couple. My sister is making these big mistakes when it comes to love. She found two men who really cared for her and would have done anything for her and just blew it. She said she has dated a few people and she is too young to be so cynical and jaded about love. She is 23 turning 24. She doesn’t have a story like my grandparents who married the first person they dated which she finds endearing. My parents also were not very experienced with dating different people before they found each other at university. I personally only had relationships with three people in my life and only went on one or two dates with a couple of others. My longest relationship was my last boyfriend of seven to eight years. My other relationships were only a few months each. I hope my sister does not end up like me heartbroken and missing the love of her life. I don’t want her to feel like i have felt these last three years. I wish she would find happiness with someone and peace within herself.