If there was a sure way to stop them hitting then I would do it but nothing we try works.
There is.
It is in multiple steps but it works.
The younger one is hitting because a) it's fun and gets a response, like having a life size squeaky toy b) everyone makes a fuss when it happens which is also fun c) the right consequences aren't in place d) their impulse control is very poor.
I have left out here e) something hurts/is bothering the younger one as generally that can lead to aggression but more likely against the person who is seen to be the source of the bother (for us that's usually something like making them put on new clothes because of sensory issues) and f) they are actively getting rewarded for hitting - my DS worked out that if he was aggressive in class he got taken out to do colouring.
You haven't specified what SEN your DC2 has but if it's ADHD then the best way to help them is get the right medication.
After that, the response (from the sibling) and the fuss (from everyone else) needs to go away. Withdraw the sibling immediately and don't respond. If in the car, stop the car, stop talking. If at home, you and the sibling go into a room you can shut the door of. Ignore all shouting and banging.
With our DS, he is quite eager to be allowed screen time, so this is contingent on acceptable/expected behaviour and "jobs" (e.g. maths homework, set the table, don't hit your sister). Your child may be different - but the language that you use helps "This morning I'm expecting you to put those books back on the shelf, not hurt DC1, and get dressed. When you've done that I will play a game with you". Pick short time frames (an hour, a morning) and don't bother with sticker charts.
And you will get worse behaviour before you get better behaviour. Read up on the extinction burst.