The purpose for the dating process is to develop social skills that will help young people find the right person to marry some time in the future. While what happened to your daughter is surely a hard pill to swallow, the process did it's job by weeding out someone that would have made her life miserable. If it hadn't been her cousin it would have been someone else sooner or later. Sadly, some people have to marry people like that to find out what they're like. In that sense, your daughter has to understand, she's lucky.
The most difficult part of it has to be her cousin's involvement. Her cousin's behavior is an indication that she cares more about a mini-fling than the feelings of her own family. I would look at the possibility that her cousin has feelings of competitiveness or jealosy toward your daughter. See if you can get that out on top of the table if that is the case.
From this expierence, your daughter should take away the importance of looking at teen age relationships with skepticism until they are proven to have the necessary bond of trust. Her cousin needs to deal with her feelings toward your daughter and learn that the glue that holds families together is loyalty, trust and faithfulness. What she did shows a trait of character that, if continued, will insure a very unhappy life. When a person cheats, they immediately communicate to the person they're cheating with that they can never be trusted.
Answered Feb 24, 2011
Edited Feb 24, 2011