Is my half sister can be illigitimate?

Question

my sister born before her father and my mom got married,when she got her birth certificate from the NSO it shows there the last name of my mom which is same last name i am using.and they said that it needs to be change to follow the last name of her father since they are married now,do you think it really need to change ? isn't is more easier if just use mom's lastname and pretend shes illigitimate ? if thats what she going to do,do you think there is nothings problem later,i am trying to get her to come in korea.

thank you and i really need help ...

hope i can have answer by this time ...

Answer 1

Sounds like a messed up situation....

Answer 2

It depends on what your Korean country asks for. In America and most countries it's a patriarchal society and most children have their father's last name. In some cases, mother's give their last name to the child and there's no problem other than it is difficult to trace the father if needed.  If a marriage takes place, and the mother has children, the children are not obligated to take their step father' last name unless they want to or if they are adopted by the father and a court proceeding takes place..Do what ever is easier for you and her.