Non-Twin Twin

When you're the older brother to a sister three years your junior and, due to development timeline differences, you end up resembling twins your entire life. Oftentimes this phenomenon results in common themes present in actual real life twins. Twin telepathy, for example, is one of these anomalies and is described as one twin being able to access the thoughts and feelings of the other twin without any signs or prompting.

Her: hey Tom, what's that movie where -
Him: The Princess Bride, for the 7th time ?!?!?!
Her: thanks! Was stuck in my head...