I asked Mr Roland Foo this very question last night.

Reference Penang Discus - Mr Shaifullah Yeng
Page 66 - Culling

My Brother in law had baby fish that looked like the ones pictured in bottom of the page with fin deformitties.

As soon as Mr yeng seen the fish he advised to seperate the parents.

Mr Foo advised that there is no chance of any genetic deformities in the F1 F2 or F3 matchings. So you can bread brother to sister three generations before you even need to worry about this problem and even after that it is like at F6 or F7 that it a 100% issue.

I would say if they breed and produce problems then split them up otherwise enjoy.

Wayne