My sister is quite a bit younger and hasn't had relationships or children. So it can be hard to feel we have much in common in terms of life stage and challenges.
But we do have similar tastes and interests so we can always find something to talk about when we are together. . We just don't talk often. I took my family to stay with her recently for the first time and that was nice. She doesn't tend to initiate contact much but she always remembers birthdays and is always very generous to us all!
I wouldn't confide in her with any problems or difficulties though.
I think we may become closer as our parents become elderly and we need to work together to care for them.
I worry about the fact that she lives alone, has never had a relationship and it's very unlikely she will have children. She is very close to my parents with whom she shares many similar interests - but they are getting older and it won't be long before they're too old and frail to go on holiday with her and be "best mate" material.
But as DH says maybe I need to accept her for who she is!
DH has three brothers and I have sibling feelings toward them. We are all in WhatsApp we chat and visit each other, they're all married with kids and lots going on. We get together at my in-laws and then one a year rent a big house where we stay and get all 8 cousins together.