Apparently the soul mate thing is exclusive to the royalty, and in noble world it is common and actually almost impossible to marry someone not related (take real world royalty/filfy rich/celebrity they are all cousins and they rarely marry outside their buble)
I think here it was just how they put them to all be close since birth without forcing the bar (like actually referencing to real world history instead of forcing a plot, like no "family friend child" would be allowed at a girls birth, if they are all high nobility of the same country they would be cousins either way, I guess you could force a "mother's friends of other country" but them it wouldn't make sense for them to be in the same country unless one of them married there what would still make them related I mean nobles has their own state to rule)
You could also make the empire to be really young, so there is still families who didn't marry each other but if the empire is old most nobility of other countries would also be related
Why did they have to make the mc soul mate her second cousin?!?!! Like come on, it’s not too hard to expect the love interest to not be a relative and someone from the family.