We can never create something that is an exact replica of the wild no matter how hard we try. Mother nature can do miraculous things with balancing, we can't. I had a friend who has been to the Rio Negro around seven times now on expiditions and it still amazes us and him at what he'll find sometimes. He told us a story about how they were in this little cove maybe 10 feet but barely six inches deep, there was leaf litter everywhere and on each leaf was a single Checkboard cichlid. They tested the water and it came up to 3.4 pH and the water was 110*F and yet they were prefectly happy to just sit there and bask in the sun, waiting for their food to come along.
We try that in the home aquaria and your fish are dead, it doesn't matter what you do, you can never replicate those results.
Back to Bettas, if they are from the same spawn then yes, it's just fine generally as it is a whole different dynamic. If these fish have never been separated then likely there won't be much aggression as if they were not of the same spawn. There could possibly be breeding which is stressful in and of itself but that's a slightly different topic than what is at hand.
Learn to stand up for yourself and not follow so much. Just because someone jumps off a bridge, you tell people "oh it's okay, I know some people do it--I won't but people do it..." telling that to someone who might just jump off a bridge can literally push them over the edge. Do you understand what I'm getting at here?
Rembrandt II, Suiché
Leopards: 1:4 Hawkeye, Luna, Astrael, Dorian, Cullen
Gargoyle: Cloudjumper and Toothless