How come I have multiple sclerosis, but nobody else in the family has it?

Genes only one part. While there are some genetic susceptibilities to ms, the true reasons while one person develops the disease and another does not remain unclear. Family history is only a risk factor, and not a definite predictor, with other factors such as nutrition, infection, and so forth likely playing a role.
Combination of event. You have a genetic susceptibility, and have encountered environmental triggers which have caused your outcomes. However, usually does not run in families as is not a genetic disorder.
Various causes of MS. Both genetic and non-genetic factors are thought to contribute to ms. A person may have had different environmental exposures than their family, or their genetic risk may reflect a combination of risks from the parents. Other family members may not have gotten that same exact combination of risks.