Reinhardt, Kircheis and Annerose (Spoilers Within)

I’m only about one-quarter of the way into the series and Kircheis has just been killed. In a video call, Annerose appears to indicate to Reinhardt that she and Kircheis had been in love, which makes Reinhardt upset. But it’s not clear why. Is Reinhardt jealous of their relationship because he is gay and loved Kircheis himself or because he has incestuous feelings for his sister? Or does he just not like the idea if being the third wheel in their triad?

Perhaps this will become clear later in the show. But, if not, I’d appreciate hearing others’ opinions.