OH I AM SO RIGHT THERE WITH YOU SISTER! :)
I have a 7 and 4 year old. I yelled way too much to the point my kids brought it to my attention. I sat down and had a family meeting. I told them "I ask you once nicely, then a second time nicely, then it is my fault I allow you to get me angry and I yell, it is wrong, however I should only have to ask you something ONCE!", it is normal for kids to see how far they can push you, when I stopped yelling and started enforcing things changed. It is nowhere near perfect I am struggling with my son now but with my daughter what worked was just matter of fact after the second time asking 15 minutes off of bedtime, period. Then it would be another 15 when she didn't do it again. A few times of very early bedtimes she knew I meant business, now the clothes are in the hamper without even asking, she cleans up her messes without me asking, she gets ready for school when asked. It isn't always perfect, my son isn't so concerned and still has issues of just ignoring me but I am working on finding a chart system of smileys for him to work through it.
All I ever say "well I am sorry you chose to ignore me, guess you will take your punishment"...whatever that is for you.
We have a saying around here too, mean what you say and say what you mean, I stopped idle threats I wasn't willing to follow through with it, instead of yelling from another room I got to their level said my instructions, made them repeat it back to me so we are all clear they heard what I said. Then well if they ignore me they get consequences.
I still yell and lose my cool, however it was a habit I am breaking and it is working so much better. If they leave toys on the floor the second time asking it comes to "well if I have to clean it up you won't be seeing these toys for a while" and it is amazing how fast they clean because I have gotten a garbage bag out and put them away for a few days! :)
You have a lot of little ones, you have your hands full, if you approach them with it being part of a family as a whole to help out then sometimes that helps.
I still am there with you! :) Hugs!