Hey OP, I have a 4 year old who started reception last year. I could have written alot of your post myself including the exasperation. I suspect he might have ADHD but again, like you, not severe but suspect he's on that spectrum if you will. It's hard.
I'm not suprised Easter has been difficult because chocolate definitely sends my DS a bit extra. He's like an unwalked dog, absolutley bouncing off the walls with energy, I try to take him out every day on his bike or to the park just to run some of it off as I struggle to be in the house with him all day.
Things that I've found help is getting out of the house every day, always greeting him with a smile and talking like I'm super happy to see him and that just him waking up has improved my day. Gamifying as much as possible like brushing teeth and singing daft songs about it.
Also following through with consequences and sitting down and explaining why everything's happening.
Saying I notice you like throwing water at your sister, why is that? In a questioning rather than accusatory way, or any other repeating behaviour when yoy're both calm. He might open up.
Also I try to avoid matching his energy and getting into a shouting match he does try to push my buttons and wants to get a reaction from me. I just now calmly say if you continue such and such I'm taking away favourite toy for a day (something that won't fuck me over too like stopping an activity and threatening to go home) and he can earn it back. (Don't get me wrong I still fuck up and end up shouting at him sometimes but it just escalates a shit situation)
Now he's coming up to 5 I think he's getting easier and school has helped (although dreading transition to year 1) he had LOTS of trouble in first term of reception, getting into loads of trouble at school, being very naughty, not listening and the first parent teacher session felt like I was getting a massive bollocking. Felt like we had to prove we were engaged parents, that try to implement consequences, feed healthily and interact with our DS, that weren't dragging up our child. However second term something clicked and he's doing much better at school and at home. (We did move house shortly before school started so lots of change probably similar to you having a new baby)
He has been flagged to the SENCO now at school but only as a "he's on the radar" sort of way. I would speak to any potential school first about your concerns and make sure you choose a school with a good senco provision just in case.
If it makes you feel better mine used to spit in my face when he was 3 and it was so hard not to lose my rag. 😭 Thankfully that's stopped. Sending massive hugs OP. I think it will get better when he's older, you're in the trenches now with 2 preschoolers.