yeah it was stated in earlier chapters that the original princess thought that prince was her "full brother" but it turns out he simply looked like her and was actually the queen's kid. she used him and his similar looks to get to the princess and gain her trust i think. so once the original princess realized he wasn't her "full brother" but just a half brother.
This is just a guess but I think the black haired prince was the mc's mom's son but with different father? Cause the Queen and King aren't even black haired and have blue eyes. So they are still half siblings. The black haired prince might not be blood related to the King. So he isn't really a threat to the crown that's why the Queen is treating him nicely. Or maybe he's still the King's son? Idk.
And maybe, MC has a higher chance to be the successor since she is blood related to the King that's why the Queen isn't in good terms with the princess.
But there's something in the last panel especially with how the Queen looks at the portrait of MC's mother. It is still a mystery sooo
that’s what it directly stated in the manhwa, that he’s not her full brother. but your theory makes so much more sense since the OG princesses’ view of things isn’t completely reliable. honestly if it turns out the way you said it would make the story even more interesting. it makes sense
kinda off topic but theres this black haired prince with blue eyes just like the MC, but calls the queen his mother, even though he looks just like the MC's mother? am I missing something?