Just get real basic about it.
Don't try and negotiate or rationalize or get too wordy or fancy in explaining things to her.
Just say, when she is sassy or doing something icky:
1) That is WRONG.
2) That is inappropriate
3) That is mean
4) that is Unacceptable.
Then tell her "That was wrong. You know that. REDO that...." and then, just stand there, tell her to redo it. And wait. And make her APOLOGIZE to you for it.
I work at my kids' school, and SO often, I see kids who, EVEN if they know they were icky... they WILL NOT, apologize. Kids these days, don't seem to even know how. They are just too used to, getting wordy explanations for "why" they can't do something. And then put in time out or scolded. But they are NOT taught, HOW TO.... REDO their actions, nor how to apologize. Or how to admit... they did something wrong or unacceptable.
Ya GOTTA make sure, to keep it simple. And basic. Make sure she KNOWS:
wrong/right
mean/nice
acceptable/unacceptable
appropriate/inappropriate
And repeat those words, if/when she does something, wrong.
Don't talk in wordy paragraphs. Just a short efficient pointed sentence.
And if she puts on a tantrum or hissy-fit.
FINE.
Tell her "That is not acceptable. When you are done, I will be doing chores. You come and tell me..." and have her apologize.
A child, WILL deflate on their own.
And then walk away.
DO NOT ASK HER "can you speak more nicely...." Just TELL her "Talking like that is wrong. You KNOW that. Redo it. Now." Then, look at her.
Be, more direct and stronger with her.
Direct. Simple. Pointed sentences. Not paragraphs.
And tell her to redo it. MANY kids, don't even know how. Nor try. NOR do they even know how to apologize. Nor do they care to.
That in itself, is "wrong."
Just teach her the basics.
Keeping it simple.
And if she is so fussy, tell her "You are fussy. You need to nap..." and then, go and do it.
All kids can be sweet and sassy and very bright and creative and active and caring the next.
But they need to know, the basics.
Right/wrong
mean/nice
acceptable/unacceptable
appropriate/inappropriate.
AND how to, redo it. And how to, apologize.
Don't get all flowery and wordy with your reasoning to her when she does wrong. Just keep it direct, and basic.
Since she is 6, she may not comprehend what is "respectful" or not. OR she simply does NOT know, what is "respectful." That is a bit more of an abstract... word for young kids.
So again, teach her and reinforce, the BASICS, as I mentioned above. Before you expect, more of her.
And when she is in a normal mood, THEN, you can chat. About what the heck she is feeling or why she is "frustrated. IF she is.
And if she is, then TEACH her how to talk.
Many kids are told "Use your words..." But, they do NOT even know... what or which "words" to use, nor how.
So, teach, her.
Role play.
EVEN if she is jealous of her sibling... it is wrong. TEACH her she can talk about... her feelings. But NOT lash out. She can be grumpy, but say it differently.
From when my kids were 2 years old, I began teaching them things like that. At 3 years old, my son could tell me "Mommy I am grumpy.... I'm going over there. I want to be by myself..." and he would go to the other room. At only 3, he could do this. Once, I noticed he was grumpy. So I told him "Are you irritated?" And he said "NO Mommy, I am frustrated...." He KNEW, his feelings and the difference between being "grumpy" or "irritated" or "frustrated." And he would TELL ME, why.
If you THINK your daughter is "craving attention" from you, then Simply... ask... her. And if she does need more attention, fine. There is nothing wrong with that. She just needs to learn how... to say it, and know that she can, tell, you.
Ask her, why she is grumpy or frustrated.
Even if they don't know right now, nor be able to express why... in time, a child WILL be able... to know themselves... and... tell, you.
If you don't know her or her moods or her cues... then ask her.... what is going on. In a calm, manner.
My son is 6.
He CAN tell me, exactly, what is bugging him. And why. He knows himself. Good mood or not.
I do not, scold or punish my 6 year old son... for being grumpy or frustrated. Because, he can express it. Well.
BUT on the other hand, if a kid is just being a brat to be a brat, then fine. Discipline is in order.
Per an adult... if YOU were grumpy and snippy and things were bothering you- would you want someone to scold you for it and punish you for it? Or, would you want them to, assist you, in that?
There is a time and a place for assisting a child with their moods. And/or for them being... just a disobedient brat.
We as parents, have to know the difference.
Sorry this is so long and I am rambling.
But wanted to add:
I have 2 kids. With my eldest one, WHEN OR IF she "complained" about her younger brother.... I would acknowledge her frustrations. I would say "I know, it can be frustrating with a younger sibling... " and that alone could key her down. And her mood. It gave her empathy... from ME. It showed her *I* understood, her.
THEN, in tandem with that, as a routine... I would EXPLAIN to my eldest, about her little brother's development. Every month or week if need be. I would tell her how he is changing in his mind and physically. Why he can or cannot do things, LIKE her. That even if she is the oldest... SHE DOES not have to be "older." I know she is a little kid, herself. But that, her little brother is not able to talk/walk/do things, as well as her. Yet.
AND my explaining these things to her, helped her... to be more empathetic.. toward her little brother.
And... also praise her when you notice she does something pleasant or nice. I don't mean praising every little thing. And say "I appreciate that...." to her. It shows her, reciprocation and, that you, noticed. Its indirect feedback for a kid. It encourages. Them.