Re: Younger brother fell in love...engaged...now regrets..
e-mail her and tell her that there are lot of differences and he doesn't think he will be able to work them after marriage, therefore it is better that they part their ways now then getting married and realizing that it was a wrong decision.
The other option would be sit with her and explain her but for that he needs to make sure that he remains calm no matter whatever she says to him.