Each house in a natal chart represents a specific area of ​​life that the elements found in the house can influence, such as planets and aspects. The third house, in particular, is known as the house of communication, siblings, neighbors and short journeys. Understanding which exact sector of these four it affects in a reading has more to do with what you read, but in general, if you look at your third house, you are trying to understand these four areas of your life. Your relationship with your siblings, how you communicate with them and the influence they can have on us is an important way to read this house. The planet that rules the third house (known as Lord of the third house) is one of the key players in trying to understand this dynamic.
The placement of planets in the third house and the sign of the lord in the third house can reveal a lot about how you relate to your siblings. For example:

The “lord” is the planetary ruler of the zodiac sign that rules your third house. For example, if Gemini rules your third house, then Mercury is the lord of the third house.
If the third house lord is Mercury and the planet Mercury itself is placed in the tenth house (career), your sibling can influence or inspire you in your career. If the Lord is in the fourth house (home), your sibling may have a strong influence on family matters or be closely connected to your home life.
Therefore, by analyzing the third house and its Lord, you can gain a deeper understanding of how your sibling relationships shape your life. This can help you appreciate the unique role your siblings play in your life and if you are going through something that could be catastrophic with them, it can also help you find a way to heal your relationship and navigate more effectively.