Dear Jack, let me state this from the onset: It’s not going to work! I mean your girlfriend’s concoction. It’s an incondite relationship and she’s being most unrealistic; she’ll end up losing both ways. Chasing two rats at once is chasing none.
From your mail I see she used to date this other guy. But because of SS genotype scare they chose to call it off. Only they didn’t REALLY call it off. The liaison transmuted into “brother-sister” relationship; they’re like “twins watching out for each other”. He watches out for guys like you. He actually believes you’ll hurt her. He feels protective towards her, like an umbrella in a typical insurance cover advert. Her core is her oblation for the coverage. And she likes the feeling. In fact she’s ENJOYING it! It’s why she proposed the uncomfortable triangular arrangement. It’s some form of sharing formula – the mathematisation of her affection in pie chart configuration.