I’m not sure how it is in small families, but in large ones relationships tend to shift over time. You might be best friends with one brother or sister, then two years later it might be someone else. Then it’s likely to change again, and again after that. It doesn’t mean that you’ve fallen out with the person you used to be closest to but that you’ve merged into someone else’s lane, or had him or her merge into yours. Trios form, then morph into quartets before splitting into teams of two. The beauty of it is that it’s always changing.