This study below suggests that there are relatively few genes that control hypertension, otherwise, if not, then finding whole families with hypertension would be unlikely. There might however be another explanation. Perhaps a common infection or dietary deficiency.

"Approximately two-thirds of patients have no known affected siblings. The next largest group, about one third, is patients whose siblings are all hypertensive. In a small group, <10% of all patients, half the siblings are hypertensive.

6We conclude from these surprising findings that hypertension is not a continuous, multifactorial part of the normal blood pressure distribution. They suggest that several more single-gene disorders causing hypertension will be found. The sibships where all members are hypertensive are inconsistent with the segregation of Mendelian genetics and suggest the selection of some genes linked to hypertension at the time of gamete maturation."

http://www.ingentaconnect.com/content/bsc/...000001/art00004