Spoiler
Just looked this up it's true, my heart absolutely shattered when I heard what happened to her brother and her mother. So I'm so glad he's alive. I'm still distraut with what happened to her mother and unborn sibling though. Like the interaction with MC and the baby in the womb was so cute, I thought we get more wholesome sibling time, and yet we will never get to see the baby being born it's was so sad
What I read from the wiki is he saw an opportunity to run away from being crown prince.
This is what the translated Namu wiki said:
"He had longed to abandon everything and run away from an early age, and when the syndicate arose, he switched bodies and went into hiding. The king personally inspected the bodies and discovered they were fakes, but he pretended not to know, knowing that Hye had run away"
I FRICKING KNEW IT!!! HAH classic body swap, I already knew it was coming from a mile away lol totally not holding on to that idea like a delulu (her unnie/mom and her unborn sibling's de×th still haunts me tho)
We know for a fact that his self-esteem wasn't that high especially when you have two younger siblings whose potential shows that they're smart and talented plus that torment from his 2nd brother and that wicked consort. Now I kinda wanna know his journey tho....a reunion maybe? hmm

please dont read if u dont want big spoiler but i've been seeing a lot of "im delusional bro isnt dead he's just hiding" statements so i wanted to leave a statement here that the older bro...is alive. (you can find it if you look up the title in korean and go to namu wiki, and scroll down to information on ihye.) (for those who dont want to look up - not including all details, he faked his death.)