Daichi had his doubts about Igawa when they first met and he seemed to be trying to figure out what this guy's after. So, if he were to leave it as is and Mana tells Daichi about them making a joint account to save for the wedding a couple months after they start dating, before she even meets his parents, he might smell a scam. So, he needed to get Daichi on his side and at the same time jealous of his sister, so that he won't interfere with the plan. Also, after the scam is done, Daichi might not want to pursue Igawa or go to the cops coz if everything comes out, it will also come out that he slept with Igawa (though I doubt they actually did; it looked more like a setup) which will destroy his relationship with Mana for good.
Ahhh it makes sense. I think it's easier for me to understand if i imagine daichi was a girl. Seducing them both will guarantee that daichi won't take any action regarding the scam. I'm really excited to know what will happen next.
So seme is a criminal that steal sister's money, but what does he gain from uke?