The Third astrological house concerns communication. The typical association with the third house appertains to siblings. I tend to think of it more as having to do with "hands". Hands can be used to speak when the mouth cannot, and so we can start to understand why communication is related.
There is also the aspect of "networking" or nerves working together. All of these things fall under the concept of Mercury, which rules the third house in the standard chart of Gemini.
Hands are, of course, mirrored to one another, and our siblings when we are young are somewhat like mirrors to us in that they are not far from our own age typically. This leads to the first "this human is not my parent" experience, and so we learn how we deal with "others" that are not us, but are us--at least in a genetic sense at first.
Sometimes we will see conditions like seizures here as well. This concerns how well the brain is or is not doing with the other hemispheres of itself.
Later on in life, this house concerns how we speak and can show what happens if we engage in too much gossip or speak frivolously in some manner.
The third house is all about "twins" and "mirrors"--and not too far distant "knowledge". This follows since if we know someone else we begin to learn something about ourselves, and vice-versa.
When this house is balanced correctly, it is rather akin to a yin yang. The circle spins, but nothing flies apart.