Will she grow out of it?

For whatever reason at this point she can’t regulate her self. This is not her fault but yes you need to discipline things like hitting because obviously you can’t accept those behaviours.
First don’t sweat the small stuff, the normal parenting that you would be on top of let go for now. Focus on these big behaviours, try to stay calm (role model the behaviour you want ) give a warning then give a discipline (5 min time out or similar ) try to stick to same discipline for consistency and keep small if you give a big punishment there’s no incentive to behave.
i personally wouldn’t discipline a meltdown as if she’s in overwhelm she’s not deliberately doing any behaviour she’s reacting to the flight or fight situation she’s in. But removing her from a situation and staying with her trying to soothe could help. If you can try to get her to lay down (to ground her) or give her a cushion or similar to hit /pull at. Talk calmly and remind her she is safe/ok. Don’t ask her to do anything she will fail. Try to find things that are soothing/distracting for her (food/screen/ toy/ ) and try to offer at the start or end of a melt down to offset it if possible. When she is calm do some feelings work with her if she is willing. Using pictures try to name her emotions with her and help her understand what she is experiencing. Look at things she could do to help that emotion (again picture form) like cuddle a teddy, have a drink, go for a walk, read a book, eventually you want to get to a point where she can pause and access her calming tools but it could take a while.
The most important thing you can do is manage her environment, try to have consistent routine- eat , sleep etc at similar times every day. ( You could create a picture chart of her day if you feel it wil help) Try to pre warn her of changes to routine/new situations. Show her visuals. Try to pre empt meltdowns, so if you can see a situation that will cause a reaction remove her or distract her. Try not to set her up to fail , so if there are certain situations that set her off try to avoid them first now (you can reintroduce gradually later) Try to make sure both kids get some 1:1 time and tag team )separate activities) if it helps. What you are aiming to do is bring her anxiety/anger down so you can work on supporting her . Look for triggers and try to avoid/alter them. Make sure she gets down time everyday to reset her self.

Is she at nursery? What’s her behaviour like away from you ?
I would speak to nursery , explain the issues you are having and ask them to observe her (ideally the senco) but yes she may hold her behaviours in at nursery and let them out with you because she feels safe to.

Was she worse with the unpredictability of the holiday? What did you do to prepare her for the holiday? Looking ay photos, social stories, visual time tables, youtube videos of the place you were visiting?

Is she otherwise sensitive (light sleeper, irritated by clothing labels, quick to get frustrated)? My son is and he used to get aggressive when he was overwhelmed at that age. You sound like you have quite a punitive attitude and perceive her as the problem - maybe try focusing on reducing stressors for her (avoiding crowded places, keeping routine very samey and calm) and see if it improves?

My son is 7 now, but from between the ages of 2 until about a month ago, he was really aggressive. It peaked at the age of 5 and 6.

I'm not saying it will last anywhere near as long for your little girl, by the way.

He just suddenly stopped being aggressive. He still talks to me rudely sometimes but I try not to take that personally because he really does struggle to regulate himself, bless him.

But she might just grow out of it. You have my empathy, OP x