When is your family est. date??

My sister is getting her first house and I wanted to get one of those family established 20__ signs done but I dont know what year to put. She started dating my b.i.l. the same year they had their first child but didn't get married til 3 years later. I feel like wedding date is the typical date but is that still correct since their family started before marriage? They weren't technically "the Smith's" until 2015 but they started a family together in 2012🤷‍♀️🤷‍♀️