They say you end up with a partner who's similar to your Mom or Dad. But I've also noticed that the charts of significant friends often mimic the chart of one of your siblings.
I don't have hard evidence on this. But on multiple occasions I've seen a BFF in a person's life share the same Sun sign, Moon sign or sign emphasis as that person's brother or sister. Especially if the relationship with the sibling is troubled or non-existent. Family forms your relationship template (for better or worse) and you're constantly trying to work through these issues. Fill the void that an absent, older brother left. Work out your rivalries with a younger sister.
This raises the question of astrology versus free will. Are we all just trying to heal ourselves through other people? Our most intimate relationships (and friendships can be intimate) are there for a reason. If you had access to the charts of every significant, non-family member in your life, you'd notice the same signs popping up. And you might notice that one or two signs were completely absent (or in the minority).
Back to your chart. If you have the information, get your sibling(s) charts and the charts of one or two close friends. What planets are in the same signs? For example, do they all have planets in Taurus or Aquarius? Restrict your analysis to the personal planets (Sun, Moon, Mercury, Venus, Mars). Now, what houses in your chart do these signs occupy? These are your issues, triggered by your sibling, that you keep trying to resolve through your friendships. What about the signs that are absent? Let's say you've never been friends with an Aries or are completely unfamiliar with Leos. I'm not talking about the signs that you dislike, I'm talking about the ones that you have no experience with. They are unexplored country. Check the house in your chart with that sign; is it an area you're avoiding?