Well you did the right thing in apologising at least.
I think this might be worth speaking to a councillor about if its at all possible.
Also remember kids are dumb. Some people get caught up in toxic behaviour especially when it goes unchallenged and they don’t have the awareness that it is wrong. At least as an adult you have found that awareness and tried to make amends.
Nothing you can really do now. You could maybe ask if there’s anything you do for him and just be open to give assistance?
I have to say, relationships between siblings can vary a lot. Some people have close friendly relationships. Some people kind of put up with each other, but don’t really like each other. Other times like with me, we don’t talk at all and if we do it’s normally for negative causes.
I just hope you aren’t painting yourself is some evil villain because you used to tease your younger sibling a bit. As I say, children are idiots. They literally don’t have the ability to know what they are doing is wrong.
I think just be there for him now if you can is the best thing to do moving forward. But don’t beat yourself up for things that are long in the past now.
Believing it is more likely to make it come true. Forgiveness is only the first step to repair. If you can’t accept the forgiveness, then you’re denying the next steps from happening. You’re preventing repair.