The planets in the Third House of the natal chart can give us clues about how we will fit into our family. It also indicates the sibling karma and can provide us with hints of the former cooperation relating to that karma.
With the natal Sun in the third house, you may have been in the previous incarnation the parent of your present-life parent or sibling. There could be some unresolved matters that came forward from that lifetime, which will create challenges within the family.
With the Natal Moon in the third house, you may have been the mother in the previous life. In this lifetime, you may find yourself being motherly towards your sibling(s) or parents. Also, you may have some unfinished business with the soul who is your sibling in the present life.
Natal Mercury in the third house indicates communication karma with your sibling (gossiping, that affected the past-life of your sibling, for instance)and also within the family in the past incarnation.
Natal Venus, on the other hand, indicates a harmonious relationship with the sibling in the past-life. For example, if your natal Venus is in the sign of Libra or Gemini, this could denote that you and your present-life sibling were friends in the previous incarnation.
Natal Mars in the third house signifies that you were most likely at war with your present-life sibling. That could well have created karma related to aggression. If you have this placement in your natal chart, you should learn in this lifetime to stand up for yourself in interactions with your sibling, especially in times of any kind of misunderstandings and/or conflicts.
With natal Jupiter in the third house, you and your siblings were connected to something to do with religion or higher learning. You may have been priests or teachers. The karma here could be related to wasting the funds or assets of the organisation.
Hence in this lifetime, if you were to work together, you must resolve unfinished matters regarding the exploitation of the company’s financial resources. This can be accomplished by being fair and loyal in business partnerships.
Natal Saturn in the third house usually indicates karma connected to responsibility within the family. In the previous life, either you or your sibling had control over another. That could have been a teacher and student relationship, for instance. On the other hand, perhaps you were siblings in the previous lifetime, where the elder one was responsible for the younger one due to one of the parents passing away too soon.
With natal Uranus in the third house, you have the need for freedom and independence within the family.
Neptune, on the other hand, often indicates that one doesn’t feel like it belongs within the family; there may also be issues related to deception and misconception with a sibling.
Finally, natal Pluto in the third house is connected to power struggles with a sibling. That is the result of karmic issues linked to control, destruction often regarding death.