I see not problem as long as they are for pork production.
But a new boar every 3-5 years would be smart.
A boar starts mating about 7 months & is good for 24 month, so every 36 months you need a another boar, brother or not.
Boar replacement is because they get to big to do the job, too lazy to do the job.
If you know another farmer, you can swap male piglets & raise your new bloodline.