Siblings may symbolise our natal third planets. Personal planets reflect personality. Sun and Mercury in a natal 3rd indicate a revered sibling. They're clever or popular.
The fourth house is synonymous with home, so the planets that fall here in our natal chart will reflect the overall energy of our early childhood years.
The foundation we lay and the guidelines we make for our children are reflected in this house's natal planets.
The native must develop, and the planet's owner will encourage it. When a planet creates an aspect—especially a square, opposition, or conjunction—we may access our former lives.
The energy is focused more on tapping into the gifts from your previous incarnation and bringing them now to teach others.
Sun synastry ignites us. In synastry and the natal chart, activating the Sun/Moon shows our familial function. Planets in the third or synastry in the fourth make us parents to our siblings.
Mercury seeks knowledge. Mercury in synastry with the family makes us eager to study, explore the history, and build a new future.
Synastry helps us understand ourselves via the person with Chiron or who triggers it. Chiron synastry affects both directions.
The Reddit poster says these traits "tend to run in families [sic], in observing how power is exerted, abused, and sacrificed within nuclear families."