You can buy up to 8 passes per 12 month period for yourself and immediate family members living with you at your address. All adults in the party must present a valid ID that proves they live at the same address as the DVC member.
The members website Annual Pass page no longer defines "immediate family". Previously they defined it as: Spouse/Registered Domestic Partner, Parent/Step Parent, Mother-In-Law/Father-In-Law, Child/Step Child, Brother/Stepbrother, Sister/Stepsister, Brother-In-Law/Sister-In-Law, Grandparent, and Grandchild.