It seems impossible for bloodline magic to remain controlled while maintaining human dignity, and marriage among different nation's has always been an important political tool. Inevitably some black sheep, a fourth son or a second daughter, will find they will have more political power in another royal family. The Twin kingdoms or even defecting to a new nation entirely seems highly likely.
This isn't unique to Zenjirou. I don't see how it's possible to maintain control when every royal going through puberty has a WMD or a philosopher's stone in their pants. I would expect bloodline mages to be treated like bile bears or pandas not royalty.