Familypedia.wikia.com was founded in 2004, using software almost identical to Wikipedia's, and started using three standard semantic extensions in 2009.
Familypedia has a well-developed system of templates, which can display on an individual's article numerous data items and tables of spouses, siblings (differentiating half-siblings) and children, with subpages for automatically-generated ancestry trees and descendant lists..Example at http://familypedia.wikia.com/wiki/Heather_Campbell_(1911-2004)
Numerous other tables are available, notably tables of people who had events such as birth and marriage in a specific locality or county or nation. Example at http://familypedia.wikia.com/wiki/Plymouth/bdm
Intending developers of a new semantic extension for genealogy can probably learn a lot from Familypedia. A new semantic extension may not be needed but could be worth pursuing. This one seems to have been going very slowly, which is a pity. [[Extension:Genealogy]] is newer and may be more likely to succeed.