Oscar always chose his own fiancée in every timeline. Empress doesn’t even know half the messed up things Oscar has done. Neither does Carcel’s father who is the crown prince’s biggest political supporter. I doubt if even Carcel knows half the bad things crown prince has done as an adult. Crown prince always keeps a clean reputation for the most part.
The empress only knew about Ines’s miscarriages and if crown prince had an official concubine which isn’t anything new. Ines even admits the empress never knew about the more scandalous things the crown prince did. That’s why she wondered if her suicide destroyed his reputation because that was her payback. She said the crown prince was obsessed with a good reputation. The empress only found out about the scandalous things after Ines’s suicide revealed everything.
As to Ines’s brother, ep81 already showed that he gave Ines a bullet and told her to kill the crown prince and if she didn’t have the courage to do it, he would kill the crown prince himself. Either way, he told her he would take responsibility for it. Since Ines didn’t want her brother to rebel to rescue her, she decided to kill herself instead.

they're both genuinely fucked up What's wrong with these devil spawns in the palace