I think I'd be uncomfortable with this not because of approaching random strangers but rather approaching them with false pretenses. You're essentially fooling people into listening to your ill-formed business plan by lying to their face about non-existant siblings in non-existant danger. Whatever happened to being honest with people? "Hi. Can I buy you a cup of coffee in exchange for some feedback? Five minutes is a…
Obviously I'm not addressing the morality of it, just the effectiveness.