K.B.
My daughter is turning 5 middle of August... Right there with ya! My daughter is a great girl but she will pull that attitude out of nowhere. It is part of them growing up and seeing what they can get or how much they have to follow what you have said.
We use to do timeouts, but have not used them for 6 or so months now. I will just speak to what I do, try what you would like.
No siblings for my daughter so can not speak of that, but what my mom did was tell use share or toy gets taken away for a few days OR she would say "alone time everyone." Alone time ment we could not play with each other, if possible in each kid in a different room... or she would pull out workbooks/coloring/reading books and she would sit two in the kitchen and two in the dining/living room for some silent time.
Wanting something but not able to have it.
That dessert or snack, testing the boundries to see if you really will stick to your guns. If my daughter asks a say no and remind her why, if she starts to whine or throw a fit I tell her I do not listen to this so go to room till you are done whining. I do not do the walking away, to me that shows they have the "power" to push our buttons, instead stand firm have him leave the space/area.
The toy, activity that can not be had or done right at that time. I give a simple reason why and if my daughter keeps pushing it I say "I said no, if you are going to throw a fit do it in your room because no one here wants to hear it."
I do not say sorry you feel that way, I say "I understand that you feel frustrated/mad/sad/angery/whatever this is the reason why you can not play/do/have this." Then if the attitude keeps going then I follow up with "if you are going to act like this do so in your room, I do not listen to this type of behavior. Once you act respecfully and nice you may come back and talk to me."