Re: Help.. advice… 
Not only young, she is NOT prepared for this commitment. It is obvious and very apparent. She is probably trying to make everyone happy that is why she is going through with this. But she is child for god sake!. Your brother will constantly face same problem if the root cause of her behaviour is not addressed. And sorry to say your underlined reasons for brother getting engaged with this cousin is not good enough!.
You mentioned that your brother is loved by your mamoo’s family..sorry NOT good enough reason. I am sure he must be shareef dude but marriage is more than all this. It is about your brother and your cousin or whatever potential rishta he will have. Have your parents tried to introduce him to other ristha’s?. Has he talked to girls who are more mature?. If the answer is no, then your brother and your cousin will constantly face same problem.
I am saying all that because, my best friend’s cousin has gone through same thing..and now my best friend’s cousin and his cousin-wife are filing for divorce after 1 year. And all the rishtay dari bharr main gaye. lol Go figure.