I'm not sure why you're confused but I'm pretty sure they explained it in the latest chapter (20) and in chapter 6.... The female main character (Jinxi) was found as an infant at the orphanage by aunt Zhang. She was adopted into the family because the purple haired sister had the aura of death, so they adopted Jinxi in order to counter it. Shes connected with the Lu family because they also have a curse, so to counter that curse as well, Jinxi is marrying the grandson.
Perhaps it's the two families' obsession with fortune telling and the supernatural that is not that easily understood? I myself am also having a hard time understanding how they can so obsessively believe in all that stuff...
Ah I understand the confusion. It's most likely since because of their culture. I'm not very informed about chinese culture, however, I think they like to rely on fortune telling (or spiritual beliefs) for wealth, health, etc. Anything that will lead them to a decent, if not better lifestyle.
I don’t understand her relationship to the family or adoption. Does anyone get it?