Many of us have a sibling, and they may drive us nuts, be our best friends, be our worst enemies, or be our best competition. What can show how we are with our siblings, and what kind of relationship we'll have? Look to Mercury, the 3rd house, and Gemini.
Siblings and Mercury, the 3rd house, and Gemini:
In astrology, siblings are ruled by Mercury, the 3rd house, and Gemini. You can be more focused on your sibling - talk with them, spend time with them, fight with them, feel distance with them - with aspects to your natal or progressed Mercury, 3rd house cusp or ruler, or natal or progressed Gemini planets.
Hard aspects (squares and oppositions) to Mercury by Saturn, Uranus, Neptune, and Pluto can often bring difficulties in your relationship with them. Saturn or Neptune can drive you apart, while Uranus and Pluto can bring power struggles, control issues, changes you don’t like, or unexpected problems.
When Saturn is in your 3rd house, you may feel more distant with your sibling, or you may have to take on more responsibilities with them. When Uranus is in your 3rd house, you may feel that your sibling has become too erratic and unpredictable, or that your relationship with them suddenly changes.
When Neptune is in your 3rd house, you may drift apart from your sibling, or feel that more compassion is needed between you. When Pluto is in your 3rd house, you may fight over power and control, and need to work on transforming your relationship to avoid it becoming destructive.
Good times to work on your relationship with your sibling or to spend more time with them usually come with sextiles and trines by Mars and Jupiter to your natal and progressed Mercury, 3rd house cusp and ruler, and Gemini planets, or with your progressed Mercury sextile or trine any of your natal or progressed personal planets (Sun, Moon, Mercury, Venus, Mars).
The Sun, Moon, Venus, Mars, or Jupiter in the 3rd house can also be very helpful, and you can bring more attention to your sibling with the Sun, connect emotionally with the Moon, meet in the middle with Venus, feel more driven to focus on them with Mars, and find a more happy place between you with Jupiter.
Transit (moving) Mercury in the 1st house (of the self) can also be a good period, and as he moves through your houses, you can focus on different aspects of your relationship with your sibling (making things more stable in the 2nd house, working on communication in the 3rd house, trying to get emotional understanding in the 4th house, enjoying one another in the 5th house, paying more attention with the 6th house, meeting in the middle with the 7th house, digging deeper into your issues with the 8th house, having new experiences together with the 9th house, finding the best part of your relationship with the 10th house, coming together as friends with the 11th house, and eliminating past issues with the 12th house).
Whether or not you get along naturally is usually found in the natal charts between you using synastry, where you compare your charts. If there’s a lot of elemental affinity (so you have planets in the same or complementary elements), this can be very helpful. I
f you find a lot of your planets square one another, you may butt heads a lot and lack understanding, and if your planets are mostly opposite one another, you may be total opposites.
The composite chart (the chart of the relationship which merges your chart with another) can also tell you come more about your relationship and how easy or complicated it is. If there are multiple planets in the composite 8th house, especially personal planets, a prominent composite Pluto, or lots of Scorpio in the composite chart, this can be the most challenging.
Helpful energy in the composite chart is seen with planets in the 3rd, 5th, and 11th houses, a well-aspected composite Mercury, or composite Sun, Moon, Mercury, Venus, or Jupiter in Gemini.
I also find that siblings who tend to be close usually have a strong 3rd and/or 4th house connection in the natal or composite charts. The 3rd house rules siblings and the 4th house rules family, so this isn’t a surprise.
Suggested Reading: The 4th House: Your Home & Family