Keep it simple... Keep the introduction simple...
Probably the two brothers and sisters started cultivating in order to survive and find their origins. They were taken advantage of by someone who raised them. Then the brother started explosive mode and the sister started falling in love mode... Ahem, in short, it is the history of the two brothers and sisters' struggle to cultivate immortality!
(Absolutely no brother-sister love! Absolutely no brother-sister love! Absolutely no brother-sister love! Say important things three times!)