How come I have cardiac arrhythmia, but nobody else in the family has it?

Not inherited. While there are some conditions that cause arrhythmia and run in families, most are not inherited. Underlying heart disease (eg hardening of the arteries, rheumatic fever), electrolyte imbalance, drugs, stress, alcohol, smoking and many other factors are involved. Rare entities: brugada syndrome, hypertrophic cardiomyopathy, prolonged qt syndrome may run in families.
A hundred reasons. It could be genetic but your other family members may have dodged the bullet. On the other hand it could be one of many issues such as caffeine and alcohol intake, too much exercise or the lack of it, overweight, sleep apnea, untreated high blood pressure, smoking, certain heart valve problems, lack of sleep, stress, electrolyte abnormalities to name a few. There could thus be hundred reasons.