He's the fourth prince and she's the 8th princess so pretty sure it's not a cultural issue. And if they are step siblings they are related which means it's still incest. It's not consanguinamory if they're not blood related however.
Yea, its frowned upon even it its not "blood", its still not morally right but whateves its got good plot
Incest is, as a general rule, sexual relations between strictly blood relatives (consanguinity as you've mentioned), and although it only sometimes can apply to relatives connected through marriage (e.g. step siblings), that generally only applies as 'incest' when, for example, the relationship is that of actuals siblings, i.e. the siblings have grown up as siblings and effectively share a genuine sibling relationship.
Just want to be clear in saying I am in NO way trying to justify any incest because I steer CLEAR from those stories believe me, but this just isn't that case at this point fortunately. If it is later revealed they are actually blood related I will be dropping this, but from what we've seen I don't believe they share blood relation. Fingers crossed :')
I haven't been able to find the raws so I'm not sure on what the actual circumstance IS, but it's NOT an incest story. She's either called him Oppa and it's been directly translated to 'older brother' or it's relation through marriage i.e. step siblings which is not incest as they're not related.
Also, MAYBE SPOILER SO LOOK AWAY
but I'm SO happy that the male lead is the fourth prince with the mint hair, Amor! I'm a little tired of the toxic villainous murderous dark prince getting the girl, and although Amor gave me second lead vibes I'm excited to see him as the lead!