I'm going to post about the wedding ring my brother bought before he could propose
Why would you post about something like this? Isn't it their private business?
He went on and on about how he would buy this for her, and she did not pay attention at him and totally did not care
It is wrong to post about other people situations. You could really embarrass him and ruin your own relation with him.
I'm gonna do it to feel better because he's totally stupid and no girl would go out with him.
He's your brother so you should be supportive of his relationship rather than using it to get back at him. Just let it go and try to improve your relation with him.