It's a social taboo and imbreeding are the only given reasons of why it is "bad".
Some could argue a failed relationship can lead to issues in a family household but I don't think that has any real merit. Dating a family member's best friend and it going sour can have similar consequences.
Some could argue a failed relationship can lead to issues in a family household but I don't think that has any real merit. Dating a family member's best friend and it going sour can have similar consequences.