
Because she is the one who takes most after the shitty emperor and is his favorite child.
Even Lart redeems himself later in the story.
Adellai never does. And the emperor only starts to be (fake) nice to Blondina because of amon (who is the future leader of the shinsu) being smitten with her.
Adellai misunderstands and commits herself into harming and destroying Blondina.

Sister will be killed when one of her plots to kill off Blondina backfires.
And Lart gets with Lucy in the end and becomes the Emperor when his father abdicates.
The father prior will try to suck up to Blondina (which aggrevates Adellai more) because he finds out that Amon is an important Shinshu - he is their future tribe leader.
Blondina sees through this and does warm up or forgive her father.
She forgives Lart tho when he apologises and tries to make amends.
I heard they (Amon and Blondina) will have twin sons.
Oh and the 2nd ML isn't a nice guy. You find out soon in the next chapters, he is another arrogant aristocrat he sees himself above Blondina. And reckons because of her "inferior" background and lack of those who will want to associate with her.
He is a shoe-in to make her "his".

She is a stupid hoe with low self esteem and is incredibly annoying.
No I am not bashing her for being a hoe.
There are plenty of slutty characters that are at least interesting
This fool doesn't even fuck around because she wants the sex. It's due to her low self esteem for being cheated on and craving male validation.
What's ironic is that she is so traumatised about being cheated on that she goes on to sleep with other men that are in relationships. wtf?

That pregnant bitch pisses me off. She dumped him when he was in love with her for another man and now that her current husband has lost his prestige she is trying to seduce ex while heavily pregnant with her husband's kid. Even using the kid's life as a emotional blackmail weapon to hang on to her ex.
Whereas, the ML needs to stop being a simp for this gold digging fake bitch. Lets not forget in earlier chapters he shoved his 'wife' which caused her to bleed, when she accidently broke a photo of the hoe ex.
The FL is better off without her 'husband'.

Was it because his sister got praised for university choice where as he got criticised by his parents?

When he go for what he wants (advertising) the parents didnt even celebrate and instead criticised him and told him to follow his older bro footsteps and go business. While his sister got praised for and even celebrated it. The unfair treatment probably build up and made him really frustrated and betrayed and seeing his sister being happy made him so bitter he couldnt hold back the tears or smth. Just the usual stuff every middle child goes through. 24/7 jealous of your siblings cus they hv it so much better than u. And doubt ur self-worth constantly want to test if the parents would care if u had killed urself. So yeah its probably becus his sister got praised while he doesnt
baby Rowena is so cute.
Sorry. I accidentally downvoted
Tf HAHAHAHAHAH