Should I bother?

I mean it would be up to the older son to try to make amends.

If I was the younger one and had been bullied by the older one, I think any real mending of the relationship would have to start with the older one truly understanding the extent of the hurt he caused, then sincerely apologizing.

I would speak to older son about attending therapy for this, and then following the professional advice on it.

I don't know if he'd want to go through all that but it sounds like the younger son does have cause to not want to be close to him at this point and it would be up to him to try to make amends.

Time has moved on and your older son is missing his younger Brother now that he's grown up and probably realised that what he did wasn't good. Maybe if he phoned him and apologised ( And it's never too late ) and ask to meet up with him, maybe go for a drink? and a catch up of what's happening in each other's world. One day they will need each other when the time moves on, parents don't live forever...

Do your two sons have a shared interest eg supporting the same football team? They could go to a match together, as a boys day out.
It's up to the two of them as adults to organise it though.

It never ceases to amuse me how many family members, and we've had our share, come out with the "you never phone" line 🙄

I think I'll leave it.Seeing them separately will do.

Wants are OK to have
How realistic those wants are is a different can of worms and a mother doesn't have any control over that unfortunately

Of course most mothers want their children to be close and it’s disappointing if they aren’t. Kircubbin

I can’t see anything wrong with a mom wanting to see her two sons and partners for a meal. It was busy with 13 people and at least you could talk to them. No need to ask them questions about their relationship, just general light conversation, if it’s a disaster you needn’t try again. You are just being a kind mom in my eyes,

Oh no don’t invite them both it will probably disappoint you
Leave it you cannot force friendship where there is none
See them equally but separately

No way you should be involved

Two adults can call each other if they are interested

It may be your dil is supporting your son, her partner, because of the bullying which went on. It really is up to them. If your older son wants to see his brother he needs to contact him and talk. An apology maybe.

They're adults, leave them to it. There's no law which says family members have to be close to one another.

Why do you think your younger son doesn’t bother with his elder brother? This would tell me if a reconciliation is likely or if it is best that you stay out of it.

I would invite them both out for a meal and measure the response carefully.

I would leave them to it too

It might be best to leave them to make their own arrangements to meet, but if you do decide to do this can you make it just you and the two sons, without their Partners. If there us animosity it won’t be comfortable.
My mum tried to do this with her two grandchildren sadly it didn’t work. They are nearing 50 now and no closer, but I suppose at least she tried.

The phone works both ways I believe. Does your son who complains ever call his brother? I would keep out of it.