Well, how I understood the story( feel free to correct me), they used to be half brothers in their first life, the uke wanted to protect the seme so he wanted to manifest as a Yin and hoped for the seme to be a Yang. But he manifested as neither (neutral), which is why his dad abandoned him. The seme was mad that the only person who he liked in the palace is gone now, so he tried everything to become a great Yin person. After a few years they met again in a festival (or something) and the seme uses that as an opportunity to have the uke for himself again. Then we can see in the story how he got r*ped by the seme and practically becomes his “brother’s” concubine. No one knew that they used to be brothers so that’s why he could become an official concubine. But after some time people start the very much true rumours that they are brothers. So everything summarised, in the first life the seme gave the uke really bad trauma and destroyed him physically and psychologically as a human being, that’s why he decided to kill himself after failed attempts to escape in the past. In their current life, they are in the modern time. The uke reincarnated as a rich master of a well off family, with a great sister and mother. The Uke starts to dream about his first life again. That is why his family thought he just happen to be a weak child. Which is partly true, the dreams made him become a bit weaker. The Uke figured out of his dreams that the person who is dreaming about every night is him in his first life. Because of his dreams he decides to be cautious of his surroundings and make sure to never see the seme again in his current life. Which he failed miserably since he met a child which reminded him of the seme while having swimming classes.the child tries to befriend the uke, but he doesn’t want to be friends with him. The child tries everything and puts up with the uke’s behaviour to make the uke trust him. After a few years he does trust that child and he becomes his only friend. Then another comes into the story which is exactly like the seme in his first life. He panicked and the child (his bsf now) comforted him. The uke doesn’t have any suspicions about him anymore and realised his feeling for him. The other guy kidnapped him and he gets bad dreams and hallucinations of the seme again. He tries to escape and the bsf just HAPPEN TO BE there just in time to save him. Hmmmmm suspicious. I only read until here but I got a feeling the bsf hired the other guy to made the uke rely on him more. 100% sure that the bsf is the seme from the first life and he is just trying to approach the uke in a different way this time.
Between the flashbacks, the kidnaping, and past life memories, I have no idea what this story is about or what's going on.