You can use your old sim or ask O2 to send you one if you need and can get 5g.
I'm sure it will be fine.
Just put your sim in the new phone when it arrives @Leah18
Your brother will need to get his own contract as your contract will stay on your sim.
Well how can I set my sim back to having 5G instead of 1G
What do you mean by 1g exactly?