Over the years. I discovered a non-atrological technique that has a success rate of just under 100%. I have found that it is extrememely easy to tell if client has siblings or not. All you have to do is to engage your client in a conversation. Nearly always, the client will mention a sister or brother in fairly short order. The only time this technique ever failed, the client had a brother that she refused to talk about. But she was so angry at me for introducing this technique that I found out while she was in the process of berating me.
This technique works, and it is non-atrological!