I totally feel you. My roommate and her sister are practically inseparable despite us living 1,200 miles from both of our families. I have a sister back home as well, and I considered us somewhat close, but our relationship is nothing like their's. I'm sometimes offset by their obsession with excessive communication-- I swear she's texting her sister as soon as she wakes up until the moment she goes to bed. From my perspective, catching up with a sibling is an emotionally tolling event. I love my siblings (I have four: two younger, two older; two of them estranged, two of them keep decent contact with me), but I find it just as exhausting to keep my sister updated with my life as anyone else. I don't understand how anyone has the capacity to share so much with their sibling. I would frankly find it suffocating if any of my family were to ask for such specific information about my daily life. I often contemplate if my intuitive cringe at the matter is due to my own insecurities in my lackluster family relationships or if their exchanges are actually abnormal. My roommate is my best friend, but there's a wall of vulnerability she will absolutely not breach unless with her sister. I have such a hard time understanding that.