So what!! Hes an ex for a reason and he blew his chance so brother wants a chance so go for it and who cares what the ex says as hes out of the picture. If the Brother is ok with it then go all out for it and let ex
Am in love with my ex's brother. So my ex he is angry with that and now I feel bad. What can I do for him to forgive me?
There's really nothing you can do because we can't control how other people are going to feel about anything.
Your question states you are in love with the brother - but how does the brother feel about you? Is it mutual or just one sided?
It is your prerogative to feel any way you choose about anything. Part of the joy of being a human being and having the ability to think your own thoughts and feel your own feelings.
Best of luck!