incest
Sex between 2 yukkuri who are paste-related to eachother such as parent-child or between sisters.
Often depicted as an accident or abuse. A yukkuri who is turned on is willing to mate with anyone nearby so when a yukkuri is accidentally (or intentionally in the case of abuse) turned on it may instinctively rape a nearby yukkuri which could be her own sister, parent or child. Often involves koyukkuris and as such usually leads to their deaths.
In the rare case where it's isn't rape, yukkuris doesn't seem to have a problem with incest and is content to mate between sisters