Responses
The crown Prince is the son of the emperor so their father
The older brother is just a temporary replacement
the grandfather is still emperor so his son (the MC and his siblings' father) is the crown prince who will become emperor eventually. the MC's oldest bro is considered the 1st prince and is the next in line for the throne after the crown prince.
Isn't the crown prince their 1st brother so how is his child running for the king title