So the one who actually was dating the chairman was the younger brother, correct?

Yeap and I guess since he's an idol they swapped so his carrier doesn't crash

but I still don't get the video, the chairman was sleeping with the younger brother when he died, then he called his older brother and the older brother got dressed in women's underwear and slept with a dead man? (⊙…⊙ )

So in order for it to seem believable that he was at the crime scene instead of his brother, he would've had to leave some kind of DNA behind. Since the crime scene is full of bodily fluids, he needed to make sure that his fluids were there in order for it to look like he had just finished having sex. He didn't actually sleep with the chairman, he just masturbated near/on him so that if they tested him, it would match the storyline of what had happened.

I hope this explanation helps

probably to make it more believable or so