Unlikely. Spina bifida (there are several types) is associated with some known genetic defects however the majority of cases are spontaneous and most cases are more likely related to folate (folic acid) deficiency during pregnancy. The general answer is that this is unlikely to occur in your nephew.
Unlikely. Most spina bifida does not run, in families, but occasionally does. Depends on type of spina bifida. If the cousin is beyond early childhood and does not have symptoms, then highly unlikely.