Help! 5yr old behaviour - really struggling

[Belle074]

My 5 year old middle child has always been stubborn and pushed boundaries at home whilst mostly behaving at nursery / school but we’ve always been able to bring her round.
However she now has a 2 month old sister and has started in year one and behaviour just seems to be getting worse. Today has just been awful and I’m in desperate need of advice and ideas of techniques to help.
Hitting, high level pitched screeching, answering back, doing the opposite of what was is asked, refusal to move when walking, extreme hyper ness, running off, not listening. The list goes on. This is between bouts of being so lovely and doing what she’s asked so she knows how to behave.
We’ve tried time outs, taking things from her, counting down, charts, Pom poms in jars. She doesn’t care about any consequence given at all.
She’s been tired after school and recently started a gymnastics after school club that I’m going to stop at half term which was extra on swimming and dancing lessons she has. I know that the year one transition and having a new sibling will effect too, and I’ve tried to involve her lots. She constantly asks for holds, which I give her, but sometimes she tells me to stop feeding and to put her in her for, so know there’s some jealously there.
I just need something to try change this behaviour before I’m in a heap on the floor. What can I do? I need to change something as I’m getting so low with it all and end up shouting. I’m scared that this will continue on with the baby, I need to nip it in the bud now to regain some control on it all as I can’t carry on as we are now. It taking over everything we do as a family.

[HappySamii2826]

Heya, honestly my son hunter is basically the same. He'll be 5 this year and he has a younger brother kai aged 1. Hunters always been a naughty boy, always extra to deal with, from a young age he was always up to no good but over the years there has been no improvement in his behaviour. I've tried all the steps to improve it (naughty chair, taking things away, charts, praising him up like mad when he's actually been good, blah blah)
He's always hurt his little brother, when Kai was a couple of months old, hunter used to come out of his room in the middle of the night while we was asleep, sneak in and hurt kai. Wed wake up hearing him running off and kai giving a hurt cry :( and even now, hunters still incredibly horrible to him. I don't trust him what so ever. I'd love to know some advice on what to do. I've been doctors, been fobbed off. I've spoken to the school. I'm doing a parenting club in january. It's just awful and so hard to cope with. I feel you! Just hang in there, and hopefully we can both get some advice x