Help with 5.5yo daughters behavior.

I am at a loss as what to do with my daughter atm, her behavior is just getting so bad. When she was younger she had alot of speech problems and it was a bit of a set back for her, she also had and still does have problems with going to the toilet and wets herself alot uncontrollably, I have also been taking her to a pediatrician to find the cause.

In the last few month it has been the worst, she refuses to do anything I ask and just shakes her head and says no, when she needs to tidy up she will just sit there and watch her brother do it, and when I ask her to help she still refuses. If I send her to her room and she finally listens and goes she will stomp loudly then slam her door atleast 2-3 times. She will then sit on her bed and stomp her feel on the walls and scream.
She doesn't behave anywhere near as badly when my partner is home. He tells me to be firm towards her but it doesn't seem to work for me.

I have taken toys/treats away from her but she doesn't care. I know it sounds harsh but I have also gotten to the point that I have packed away her room as punishment for misbehaving and not cleaning her room or breaking toys or breaking ornaments. she steals food all the time, off benches out of cupboards and even off the top of the fridge. If its edible she will take it.
Please dont tell me this is normal for child her age as I dont think it is.

I also have 2 other kids, one younger than her and one older, when she is by herself without the other 2 she is normally very well behaved and will help out and listen, but when they are around she isnt. I spend alot of time with her when she isnt at kinder so it cant be that she isnt getting enough attention.
I have tried to explain to friends what her behavior is like to get advice but they dont understand or see how bad she gets as she is always different around others at sleepovers or play dates.

I need help before it becomes worse as it has gotten out of control and I feel like im losing my grip on parenting.
I understand everyones parenting is different so please dont criticize or judge me as I am trying my best.