So she is five and with a high likelihood of ADHD and you are calling her a selfish brat cos she is struggling being out of her normal routine??
I think you need to take a big step back and look at the whole picture.
I am ADHD and so is my son (well technically he is still waiting to be diagnosed, but there is little to no doubt) 5-7 was a tough age for him. We had so many meltdowns and he is the sweetest, kindest kid when regulated.
I think you need to chant the 2 important child behaviour phrases over to yourself.
She isn't giving me a hard time, she is having a hard time.
And
All behaviour is communication.
Whenever we tried to do something nice with our kids at that age, their dad would get so stressed that it wouldn't go down the way he imagine, with happy smiley grateful kids all day. His response annoyed me more than their behaviour half the time, if I am honest.
Children aged 5 aren't developmentally capable of consistently choosing to do what is better for others over their own wants (that doesn't mean they can't do it ever) ADHD children are even less able to curb their impulses and struggle being out of their normal routine.
When my sons behaviour was difficult at that age and he was outwardly seeming to act like a little sh*t the ONLY thing that worked was for me (and believe I tried a lot of things!!) Was to stay calm and quiet, keep instructions really short and simple, cut out all threats- they don't even register when they are in the emotional/disregulated state and young ADHD kids also don't respond to anything that isn't an immediate reward or consequence. Ie if you don't stop we won't go swimming this afternoon, this afternoon is not right now, so they don't care. (Though they will be upset when you remind them of the consequence in the afternoon and then you will have upset all over again). If my son was upset enough to go full melt down I would take him aside or to his room and say. When you are ready we can have a cuddle and then we would sit...if he seemed calm but hadn't come to me, I would aks if he was ready for a cuddle he would either shout no or scoot closer depending on whether he was back to calm or not. Eventually we would have a cuddle. Only AFTER he was full calm would be discuss the behaviour or what led to the meltdown and put in a consequence. Connection before correction.
"When you are calm and talking kindly we can go to the beach" and wait...remind if necessary.
"If you can't stop being mean to your sister, I will have to take you back to the room and there will be no more swimming"
All kids need clear, consistent boundaries ADHD kids doubley so. But if you deliver them with stress and threats and criticism, they can't regulate themselves and correct their behaviour as it it a problem with "doing" not "knowing what they should do"
I hope that helps and doesn't come across too preachy.