i have 3 girls. adults now 28/25/20. when little and growing up they argued all the time or fought. i never allowed them to hit or say a bad word to each other. it was constant to the point where i was having doubts about my parenting skills. so i asked a fond member of my church about her perfect children and to my surprise her answer was 'they fight all the time'.
with my adult children now, they tell me that i always corrected the wrong one, lol. the loudest which is the one i heard would get in trouble, but it was the quiet one that was the instigator. so be aware of that.
my girls get along great as adults, always helping each other out. you cant always treat each child the same in regards to decipline. some are more hard headed while the other is fragile headed. the feed back is on that is you'll be accused of favoritism. but only you kno each child mind set. my youngest is bi-polar so of course i dicipline her differentely than my older two.
back in the day, my husband said, if one got in trouble all would get a spankin'. lol its a non stopping battle, just continue, like a broken record, to have them stop.