come on.. if this is similar to the past royalties in our real world, incest is not an issue. royal marriages were mostly like that marrying your closest kin then having deformed children. it's just disturbing but it's historically correct i think
If they were going to make it historically accurate then she as a princess would never be wearing that type of modern gown. Also the royal colour at that time was actually purple and not as what most think as red. Though you are right with your statement I think the author just has some issues. Especially considering she is a commoner. Realistically she would have never been able to become a princess ╮( ̄▽ ̄)╭
since when the art style changed?