It's understandable that you may feel frustrated or think it's unfair that your older brother gets his own room while you share a room with your younger sister and occasionally with your other sister when she comes home from college.
The concept of fairness can be subjective and can vary from one family to another. There are several factors that parents may consider when making decisions about room arrangements, such as the size of the house, available space, and financial constraints. Additionally, the age and gender of the siblings may also play a role in determining room arrangements.
While it may not seem fair to you at the moment, it's essential to remember that families often make decisions based on what they believe is best for the family as a whole. If you feel comfortable doing so, you could have an open and honest conversation with your parents about your feelings and concerns regarding the room arrangements. They may be able to explain their reasoning behind the decisions and possibly consider alternative arrangements if feasible.
In any case, try to focus on fostering positive relationships with your siblings and finding ways to make your shared space enjoyable for everyone. Personal space is essential, but the bonds you build with your siblings and the memories you create together can be equally important in the long run.