My DD2 is very similar. Only she is a very loud child in every way. It is hard..darn near impossible to do the right thing. You can't just give in whenever she asks. You know that.
I also feel bad for DD, but she is sooo demanding. I have started to wait til she calms down (& I do) & explain to her that when I say no, its no. That simple. Crying won't change it. Of course it goes in one ear & out the other, but hopefully one day it'll sink in.
Not only is she loud & demanding, but so rowdy. She is constantly holding onto the little boys, who are all walking, or picking them up, or just getting right up in their faces. She is clumsy, especially around DS & she also just gets louder & louder til you give in just to shut her up!
The only advise I have is patience. But I don't have much of it myself, so I'm pretty sure you are having trouble with it.
Just know that you are not alone. I too smack on occasion (usually when she refuses to stop annoying the little kids) & yell & scream (thinking it will make her hear me ...maybe even listen).
This too shall pass... If it doesn't we can go cry & rock in a corner together