The police suspect a teenager named Joe of planning to murder his ex-girlfriend's entire family. The evidence is locked in Joe's computer. Failing to hack into the computer, the police find Joe's best friend Billy and gather some information:
1) Joe has put his ex's family members into 2 groups on a whiteboard as below:
Father Sister Son Daughter Children Cousin Nancy (her cat)
Mother Brother Boyfriend Husband Parent Sibling Bobby (her pet snake)
2) Joe has used one of his friends' names as the computer's password. The names are:
3) Joe has told Billy that his computer's password shares the same attribute that he used to categorize his ex's family.
4) A single failed attempt to login to Joe's computer will cause the computer to reformat, thus losing all the evidence.
It turns out to be a very simple logic and a smart detective quickly solves it and arrests Joe. The question is: what is the attribute Joe used to categorize his ex's family, and what is the password?
EDIT: I should have mentioned that the attribute helps to distinguish the password from the rest of the choice
The Attribute does not necessarily relate to a word's spelling / length. Even the description of the ex's pet fits into the attribute it shares with the group.