1: Your family members already have shared "secrets" if they communicate regularly. It could be pet names, terms of endearment, shared experiences, unique monikers for things. It's language that is already familiar and you already use quite often. You should leverage that, and rely on that familiarity in a crisis, rather than trying to contrive something special for crisis-only ID. The attackers' greatest weapon is your own confusion, your own willingness to believe, and creating a sense of urgency. Your attackers' 3 greatest weapons. Don't panic.
2: My maternal grandmother was widowed and lived alone for decades. She had certain ways of knowing things. For example, every time we'd come home, she would test the doorknob. If some stranger had come and tried to jiggle the doorknob, we could tell by its feel. Just a simple mechanical giveaway. When Mom and/or Dad came by, they rang the doorbell by a special pattern. It wasn't complex, but it was distinct and recognizable from inside. It wasn't a securely encrypted ID, just a "secondary ring" that was unlike a stranger's touch on the doorbell button. And, of course, my parents can always interpret the antics of their indoor cat, in regards to who is approaching the house by car or on foot...