The girl is my age and she's so ugly. I hate her and were not the best of friends. She thinks she's cooler than everyone and nobody likes her. But I think she's just a rebound girl since he had a terrible break-up. And I'm afriad to tell him this what should I do before it gets to serious

Honestly, it's none of your business, so just leave her alone and stay out of her way.

Whether you like her or not it immaterial. His girl friend is none of your business or concern.
Whatever there is for your brother to find out let him do it without help or input from you.

She's ugly and no one likes her?

Your brother must think she's pretty and like her.

You could tell him what you wrote here,but I'm betting he won't be too impressed at what you have said about his girlfriend,or you could say nothing,trust your brother is smart enough to figure out his own relationship.

You don't have to like her,but you could just be polite and pass yourself.

Let the relationship run its course.

You know you could try to be more friendly no one says she has to become your best friend, but it would show you as being more of a grown up if you were to be polite and civilised with her.

No matter what she is, or is claimed to be she's your brothers g/f and if you care about him and nothing more, you could try to be a little friendly towards her, you never know she might be a really nice person.

Give her a chance, and show them you're the mature person.

Please don't listen to gossip or rumours that's usually all invented by those spreading the rumours and based on lies or jealousy.

Im sure you're not like that...

She's not your girlfriend, so its none of your business. Leave them alone!!

Remember all, this is coming from a 13 year old.
https://www.askmehelpdesk.com/teens/what-should-you-do-if-your-boyfriends-cousin-likes-you-490155.html

That may be the excuse for acting like a brat but she needs to know it's ugly and uncool.

Jaci, I don't know if you are going to read this. However, here it goes:

How old is your brother? Do you know her from school? Does that affect what you think of her?

How much do you like your brother's ex-girlfriend? How much does that color how you feel about this girlfriend?

I can understand that you are worried about your brother making another mistake, however, it is his life and he has to make his own mistakes and learn from them. Just as you have to from your own. It's a pesky life lesson. It doesn't have to be any harder than you make it.

Don't allow your anger to blind you from getting to know someone who may not be as bad as you think she is.

Leave your brother and his girlfriend alone. You love him and want him to be happy don't you?

It's his life and his choice not yours.