A consideration of the reasons why one becomes attracted to someone who became a step-sibling through their parents' remarriage... ~I've decided to reconsider my engagement~

It appears that my fiancé has fallen in love with the child of my parent's new spouse. As I lamented over this change of heart, a close friend who had returned temporarily from studying abroad offered their analysis. "Don't you think humans have a tendency to be attracted to things they don't possess?" "What do you mean by that?" "Just what I said. Well, to put it plainly... that's it. Frankly, isn't it possible that your fiancé shares the same taste in women as your father?" "Huh?" "It's common for parents and children to have similar preferences in what they like, right?" "If you wish to dissolve the engagement, I'll lend you a hand." "In the case that your fiancé is truly a despicable scoundrel. After marrying you, he could impregnate his step-sister, declare that child to be the heir instead, and neglect you and your children. Well, it's just a possibility, but... given the current situation where your fiancé is infatuated with his step-sister, you should at least anticipate the worst-case scenario. It will be too late if things turn out that way in the future, won't it? If you're going to cut your losses, now is the time." I have decided to reconsider my engagement. The setting is vague. Published on Alphapolis.