Bad behaviour

PS My DH was 3 years younger than his brother and they never got on, to their mother's sorrow.

I'm writing from the mum's perspective. You obviously want to be involved with your grand children which is great, I think the value of this is underestimated.
Children have been through a lot recently - and they don't have the ability to see the wider picture and grasp how long it may be at home, without school, without friends. Some may love it, some are really suffering. And they won't be able to express it as missing playing with their friends, and the structure and predictability of school. They express it in the most unloving of ways - what is seen as bad behaviour and fighting etc. Parents, and in my case just me, have been the one stop shop for all social, emotional, educational and feeding/watering needs. It is extremely hard to replicate what they normally get. School reinforces the boundaries that parents encourage at home, their brains are busy and stimulated by the learning and the social interaction. They have a sense of purpose, not these endless aimless days.
Children now need time to adjust back to all this and people need to be patient with them and guide them in the most positive way possible. Yes it's uncomfortable, but imagine being that child and that parent, knowing that your child is becoming more and more disconnected from everything that felt normal and safe, and knowing that you have no options to change it.
The more patient and gentle you are about this, the quicker you will be rewarded with connection and special moments again. Any blame (real or perceived) or tension will only make it harder to reconnect (speaking from experience). Some things that could help - try and see the children separately to get some quality time, read with them, get outside for walks. These are all things that have helped my children. Mine have been much better since starting school again for a few weeks, generally calmer and want to play together again as they have the chance to miss each other.
Anyway, I hope this makes sense. I just think the unseen damage of all this is huge and people expect children to carry on regardless. This is when they need you to be there for them the most.

My lovely grandson (10) has developed a sort of social anxiety, depression and acrophobia as a result of lock down which we are slowly trying to reverse. With him he is inclined to have tearful out bursts and be less inclined or cheerful about helping around the house or with his little brothers.
We have talked it through, and he says that he has really struggled with not seeing his cousins or aunties, nor any of his friends.He has also missed not having any time to himself (trapped in doors with his two tiny siblings 1 and 3) school and seeing friends used to give him a bit of a break from their high energy play! But they weren’t even going to the child minder either during lock down
My point is that children’s mental health has been effected by lock down children are creatures of routine and generally very sociable both these factors have been eroded by lock down. Give them a chance to regain their equilibrium. My other granddaughters 8,8 and 6 have been the same!

I expect the older boy has probably always been quite rough with the younger one but now he is a bit older he gives as good as he gets. Boys often play fight, and sometimes it can be quite rough. You may be finding them a bit of a handful but try to overlook their more challenging behaviours and enjoy all the wonderful aspects to them. Believe me, they grow up so quickly and childhood vanishes. Once they are back into school/nursery routine again that should tire them out a little bit.