Hello Forum
I'm sorry to ask such an elementary question but I 'd be grateful if anyone could answer my question.
I received an email from my sister who lives in a different part of the world. Reading her question, I decided to do a horary. But which time and place shoud I use, hers or mine? I remembered having read on this website and elsewhere that the chart should be cast when the astrologer understands the question. But this argument seemed to me to rest solely on the fact that whether the person is able to read the chart or not and because the chart using my sister's time and place seemed to be descriptive of her situation, I didn't bother to cast the chart of the moment I read her question.
But since the future events/development indicated in the chart didn't happen, I'm begginning to wonder if I should have looked at the chart using my time and place. I didn't record the time so I can't compare the charts now. Anyway my question is, if that's the case, should I take the AC as myself(though I'm not involved in the situation at all) and use the 3rd house as my sister's significator or can I sitll use the AC ruler as my sister?

If you are acting as an astrologer, and your sister has asked this as a horary question, then you draw the chart for the time you understood the question, using the co-ordinates for the place you are at. Your sister, as the querent, is assigned to the ascendant. As you say, you are not involved in the situation, your role is to understand it, make a judgement on it, and offer some advice towards a resolution.