We know that cases of cousins dating and step-brothers dating step-sisters are household secrets in many Nigerian families where Inbreeding is practiced.

Long-term psychological sequelae of Inbreeding include depression, anxiety, psychiatric hospitalization, drug and alcohol use, suicidality, borderline personality disorder, somatization disorder, and eroticization (Schetky 1990; Silverman, Reinherz, and Giaconia 1996).

Read more: Inbreeding - Effects On Victims - Gender, Family, Development, intimate, and Experience - JRank Articles http://family.jrank.org/pages/847/Inbreeding-Effects-on-Victims.html#ixzz1gRaidlyK

Abnormal relationships cause behavioral disorders and the victims sooner or later lose their self-esteem and may become easily irritable in relating with others.

If your boyfriend or girlfriend shows signs of sudden mood swings of being happy and suddenly becomes depressed, don't ignore these signs. Because they are signals of deeper psychological problems that could do more harm than good to your love affair or even marriage.

I recommend http://www.psychologytoday.com/blog/mood-swings/201007/the-power-words-the-disorder-disorder.

In developed countries, such cases are not often hidden, but reported for early interventions by psychotherapists. But in Nigeria where even the worst cases of r-a-p-e are the best kept family secrets, they go unreported and the unfortunate victims bear the scars of the trauma to avoid the stigma of being ridiculed and then later in life unleash their anger and pain on others.

Why did he slap her over a little argument?
Or why did she scream and smash things when he did not give her what she wanted?

This is what I call the Angela Syndrome after the psychopathic personality of Angela in the popular Mnet's soap opera Tinsel. 

It is the syndrome of an abused victim in his /her childhood or adulthood, but left without treatment.

Dating your cousin or dating a step-brother/step-sister will have negative effects on your life in other relationships.
The sooner you seek help and treatment the better you would be.