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:
Group 1:
Father
Sister
Son
Daughter
Children
Cousin
Nancy (her cat)
Group 2:
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:
- Billy
- Lilly
- Kelly
- Ally
- Nancy
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
Hint #1:
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.