“it was a mighty nice family and a mighty nice house” what literary device was used ?

I would say it a paradox
Paradox: The term Paradox is from the Greek word “paradoxon” that means contrary to expectations, existing belief or perceived opinion. It is a statement that appears to be self-contradictory or silly but may include a latent truth. It is also used to illustrate an opinion or statement contrary to accepted traditional ideas. A paradox is often used to make a reader think over an idea in innovative way.
Hope I helped!