Someone said :
I'm meeting my friend from earlier in this thread
and his sister and the whole family tomorrow. If the situation arises I might ask her out on a date but I'm making no hard and fast plans. I'm so weak willed
That is more than just "local".
Make sure you balance the value of the friendship, with the possible outcomes.
Is it worth it?
If so, don't let it be a surprise to the guy who might feel used or disrespected.
I am not telling you to ask him for permission, but maybe to express your interest in his sister in a manner that gives him a chance to express his opinion, and maybe even prepare the ground for you if your intentions are noble.
If your intentions are only transient, then pass your way without a second look back.