Term 'enstranged' was not used in pre-fb days.
My brother (older than me by 12 years), however, back in the 1990's. just disappeared from our lifes. Relationship between him and our Dad had always been bad - he had been born 12 months after Mum & dad married and my Mother had rather pushed everything and everyone else aside. My Dad was only 22 at the time and was not happy at sharing his new wife with a baby. As his own Dad had died (Spanish flu) when he been just 10 years old, so he had no real experience of what a father should do. From what I heard from other family members many years later he was far too strict a disciplinerian.
By the time I was born, it was a completely different. ALthough Dad was always the disciplinerian, it was never too much with me, and was delighted in having a daughter.
He and my brother never got on in adult life, and one of the last things my Mum said just before she died in hospital was to my Dad to 'make it up with J....'.
My Dad, him really tried, But within four months my brother had done is oft-repeated trick of borrowing money from our 80 year old dad, and disappearing.
I did track him down at one time about seven years later, Dad had long since died, but it was made quite apparent to me that my brother did not want to know.
Out of the bluem, five years later he contacted me - in what he thought of a jokesy phone call. It was as if we had only seen each other a couple of days previously. Sadly, his relationship with a lady had ended with her death and he was feeling lonely, suddenly remembering he had a sister.
For the next few years he was a constant presence, never going more than two days before telephoning me - forgetting he had been away for eleven years.
Indeed it was my eldest daughter who actually walked down with him to the operating theatre when he was unwell. He never came out of that anaesthetic.
I do find this sort of behavior strange, not saying you need be close or best friends with your AC/parents, but no reason to make any sort of official declaration.