Imo the most important factor of why heechan could possess taekin's body isn't the fact their family background is similar, but their souls actually resemble each other, like despite the differences in their personalities (heechan is aggressive while taekin's is timid), both of them like to resolve things in their own way no matter the risks and they also like to bottle things up when it comes to complicated feelings; Heechan himself is slowly realizing this as he got used to living as Taekin
Imo the most important factor of why heechan could possess taekin's body isn't the fact their family background is similar, but their souls actually resemble each other, like despite the differences in their personalities (heechan is aggressive while taekin's is timid), both of them like to resolve things in their own way no matter the risks and they also like to bottle things up when it comes to complicated feelings; Heechan himself is slowly realizing this as he got used to living as Taekin