I have an identical twin brother, my brother is blood type is B and mine AB, how can that happened?

It is more common for fraternal twins to have different blood types since they come from different eggs/sperm. Identical twins come from the same egg and the same sperm and so share nearly identical DNA. For identical twins to have different blood types, something needs to happen to one twin’s DNA.
Small changes in DNA can and do happen so identical twins do have a few differences. What makes them having different blood types so rare is that the changes have to be in just the right place out of over six billion different possible places. The odds are very much against this happening but still, it is possible.