My coworker slept with my brother and I can't help but be upset about it.
Did you discuss with your brother that you are upset that he slept with your co-worker?
Yes, he said that he loves her now.
He didn't do anything wrong though. It's not right for you to butt into your brother's relationship unless he did you dirty which he didn't. If you interfere it might cause tension in the relationship between you and your brother.
I get what you're saying, but I really think that he shouldn't be dating her.
There is no justifiable reason to interfere with your brother's relationship. I recommend focusing your mind on something else and let it be.