she did end up working for me for some 15 years in the 1980s
That's quite a long time! It makes me wonder why the two of you broke up contact. But then I know it's a very personal question :)
Though we have a somewhat similar story I never got to know my half sister and ask myself what kind of life she had and if she still is alive. Very strange, in a way.
Strange indeed. If still alive your half-sister likely wonders the same thing about you.
The 2 of us were never close, even when she worked for me it was in a workforce of over 50, so we didn't have much personal contact. After my father died I moved out of the city so it's just a card or a mail once a year now.