Bertia wanted to follow the original route but everything was already shifted by the time she willingly and cheerfully announced her villainess plan to Prince Cecil in chapter 1 lol
Anyway, don't worry about their son's sis complex. His father is sane enough not to let him go far the incest hole.
so happy that he was aware and say protect and the son turns out great like he only lets his sister marry someone who strong and is willing to protect her like he is to her
Basically, she already liked her but she wanted to be so normal that she hoped to be with a man version of her instead.