I have to confess that I quite didn't understand this part. I think the crown was given to the next in line, which is a member of an another aristocratic house who were a "reserve" in case all the members of the main branch were unable to fulfill there duties. But I am not sure. It is possible that Dean's sister married someone which made him the king by default.
The first link is for the english version of the novel. It has not been completely translated yet. The second link is for the japanese version.
https://www.novelupdates.com/series/common-sense-of-a-dukes-daughter/
https://ncode.syosetu.com/n1337cn/
Hello everyone! This is for those who really can’t wait for the manga nor for the translation of the novel to know the rest of the story. I am going to write the full plot starting from the « Church » arc so think carefully before reading the rest!
So, after coming back from the Capital after the party with the queen, Iris goes back to the fief to continue her work. She understand more and more that she is falling in love with Dean but she is terrified of trusting someone again and she think that they are too many obstacles for them to be able to have an happy marriage. Dean, who is actually the first Prince, is also really in love with her but he told his sister that he don’t want to trap Iris in the castle. He had an horrible childhood being almost killed by her step mother after she poisoned his mother. His father, the King, was so in love with Dean’s mother (the former queen), that when she was assassinated he became a mess. He even married the one who killed his wife! Going back to the plot, Yuri and the actual queen plot against Iris with the help of he church, having her being excommunicated. It almost cause a riot in the fief and a lot of Iris’s worker quite their job. However, she was able to prove that she destroyed the church with the knowledge of the priest in charge, that the land was actually illegally sold by the Chuch and that their was a lot of corruption going on in the Priest’s institutions so it ended well for her. She had to reveal her true identity has « Alice » and was a bit afraid to go visit the town again but everyone apologized for not trusting her. Then their is the « mafia » arc when she has to deal with Dida’s past and a shady organisation. The knight who hold her down at the begging of the story came back and apologized. Then because of natural disasters and economical problems of the Royal family management of finances the taxes rises and Iris is asked to ad other 40% of her fief food production to the Capital, which she refuse by saying that she will only shoulder such a burden when the other fiefs would too (she had investigated and found out that only her fief was asked to give such an unreasonable amount). Then she is asked in marriage by a foreign’s Prince but refuse. The situation got worse and worse until war and rebellion started in the kingdoms. Finally, the second prince is killed and Yuri arrested has a traitor and sentenced to death. Iris is heartbroken when he found out that not only Edward but also Dean died. Iris has a last talk with Yuri when she show her true colors. The most important of all is that Bern, Iris’s younger sister, refuse to take the fief from her by claiming his birthright. Instead, he ask her if the wants to become the really governor. She accept and go back to the fief. Some time pass and, guess what? Dean comes back! He actually faked is death because he wanted to be free from the throne. He then declare is love to Iris and ask her if she would accept to be with a commoner, since from now on he would be « just » Dean. She declare her love too and after a few trial (a misalliance is not really common never appreciated by the noble society) she got married and have too children: a son who like like Dean and a daughter who look like Iris. Tanya end up pregnant too and married with Dida.
Happy ending for the rest of them. Iris is remembered by history for being a pioneers in a lote of reforms that would be used in all the countries of the wold hundred of years after her death and for having a love marriage rather than a political one.