>>>I have a half sister who I didnt know existed and was only seen once , and that was nearly 60 years ago. Why would I sit shiva
In such a case I wouldn’t be menachem avel if there was a major inconvenience to me. My SIL had such a sister. I’m not sure if she knew of her sisters existence or not but I certainly didn’t. No mention of her having a sister was made when she married into the family or any time later.
When that sister died I was not menachem avel. It would have been a whole day shlep and I felt that if her family never acknowledged her existence during her lifetime I don’t have to do so after her death.