“坚持”has a similar meaning with“保持”. So it can be more suitable to keep just one.

“安静” means a silent environment in chinese. In this sentence you must want to express that your sister didn't want to be angry.

So "冷静” can be more correct in this way , which means a silent emotion rather than silent surroundings .

Secondly ,"吵架”means that both of them are willing to abuse each other. So this would be a contradictory expression based on "your sister kept silent."