Can anyone help me fix this awkward sentence? I thought everything would be better with the birth of my brother, but I was wrong.

When you are reading sentences outside of the context, sometimes they can be considered awkward.  I am thinking that you mean that everything in your life would be better after your brother's birth, but you were wrong about that.

However, the reader could interpret the sentence that you have written to mean that your brother's actual birth went wrong. In order to avoid awkward constructions, I suggest that you read sentences out loud to yourself and to another person who is not familiar with your topic. If they furrow their brows and make a confused face when they hear your sentence, they did not understand your intended meaning.

pohnpei397 has given you some excellent possible revisions for your sentence.