Happened to me, kind of with two family members.
First, my dad, who was never a good father figure to start with but amongst the very emotionally fucked up things he did was when I was very young and we were legally made to attend weekly Saturday visits with our dad in a social services facility. He attended the first one, showered us with gifts, told us about the flat he had bought and how he was preparing a bedroom for us (me and my younger brother), he then promised he would be back to the next one and we would celebrate my brother's birthday. He never came back. And because we were legally summoned to attend, every Saturday for MONTHS we were made to go to that room in that building and wait on him for at least an hour each time. It was dreadful. He showed up YEARS later on the doorstep with a baby in tow and a new wife, wanting to ''catch up'' conveniently soon after I received a FB request from the daughter of his new wife saying my dad had tried to sexually abuse her sister and was beating up her mom/his new wife (all accusations I know are true). He tried to sue my mom a few days after his visit for custody ( he wanted the money he could get for being recognized as the parent of 3 kids) and then told us he would come back and burn the house down in our sleep. I had him blocked on all social medias but finally got curious enough and unblocked him (though he can't see my page) and it's very clear that on top of being an awful human being he is also very mentally ill, he had found pictures of me and my siblings (including siblings that aren't related to him) and had been posting them on his wall, and was also calling himself the new Christ and his Facebook mostly consists of him writing very cryptic and deranged stuff, mostly related to god (though he is God in his world). And I now regularly check on him, mostly to check if he is dead yet (my hope) and to keep a trail of all his erratic posts so as to use them against him if need be.
The other person is my step-sister, non-biological daughter of my dad but ''adopted'' (his name is on the birth certificate) by my dad at her birth. I was too young to grasp her story and then she disappeared so I don't have the full story, but I am pretty positive my dad did what he did to us to her first, probably left her mom one morning with no explanation and went with my mom and came back a few years later with two kids in tow (me and my brother). Anyhow, we saw each other a few times growing up, at first with my dad and then I think our mom allied to sue my dad so we would see each other then, then we stopped, and it's only when one day I was walking around the city as a teenager (so probably a decade after we last saw each other) and she somehow recognized me. We caught up briefly she explained a bit her side and how hurt she had been by my father and how she had looked for a bio father but he didn't want anything to do with her and how she had just gotten out of a toxic relationship where the man was beating her but she would say '' I fought back." I remember being very impressed by that sudden sister who seemed confident and strong, I only had brothers at the time so I was happy we had rekindled but very shortly after us rekindling, she met a new man, a muslim man, and very quickly she totally changed, her FB went to ''do not accept men'', she ended up selling all of her clothes, started wearing the hijab, then the burka and eventually the niqab and went from feminist to ''women are born to serve men and be good wives and women who don't submit or uncover deserve whatever come their way." It was abnormal. I tried to talk with her about her sudden change (it was in the middle of the ISIS conflict) and she pretty told me to fuck off and that I couldn't possibly understand since I wasn't Muslim (neither was she a few months prior), then one day, all of her social media were gone, and since then all information about her online, even things that are accessible for any citizen such as exam results that I used to be able to access, are gone. It's like she never existed and it's not like her name is Anne Smith. For good or for bad we have the same last name and our last name is unique. If you type it on internet you only find me my brother and my dad (and before her), nobody else. (which is strange in itself and probably require investigating) And now all the old information about her, including archives by the city hall is gone. It's like she never existed.
Like I said she disappeared in the middle of the ISIS conflict (so almost 10 years ago). To this day I continuously wonder if she is alive and with a family living a pious Muslim life, if she realized she was being brainwashed or abused and escaped or if she did the stupid thing of going over to Syria and died there or is currently in a camp somewhere. I still have hope she will come back one day and tell me her story, and if not I intend to investigate once I have enough resources to hire an investigator. unfortunately as her step-sister she barely knew I have very information regarding her and I was heavily relying on our last name making it always easy to find her but I realize it's like she married the guy and changed her name. Very strange and very sad. I don't think about her often, as life continues but when I do I always wonder where she is at and what she is thinking about and if she is not alive anymore, and like I suspect, went to Syria. If before she died, she regretted her decision and wished she had taken a different path. I hope she is okay and I always feel guilty about continuing to live a semi-normal life when nobody knows what happened to her and whether she is alive or dead.