Disclaimer: In what follows I shall use “[HS]” as shorthand for “half-sibling” so as not to reveal genders not germane to the story and such.
My [HS] got way too curious about genealogy a while back, to the point of paying for DNA analysis for everyone left alive in “our” family, including our mother, the two remaining brothers of my dad, and quite a few cousins. I warned [HS] that there were likely skeletons that might be better left undiscovered. Did [HS] take any advice from me? That would be a “no”.
[HS] called me a few months later and told me that 23&me (or whoever) had informed [HS] that the two of us were not siblings, and likely were no closer than second cousins, based on the fact that [HS]'s DNA disclosed precisely zero in common with my dad, while mine left darn near zero doubt of his paternity with respect to me. [HS] asked me if I knew how that could be. I thought, and replied that since [HS] has three children (with two different spouses) that it really shouldn’t be difficult to figure it out. [HS] was in the early 60s at the time.
The outcome is that [HS]'s biological father was found, in his late 80s, and [HS] was able to meet him and his entire family (including his wife, who was less than pleased with the whole production). Our mother, then in her mid-80s, eventually admitted that she and [HS]'s father had been “sufficiently intimate” during the period after my dad and mother were separated prior to their divorce to bring [HS] into the picture. [HS] has sort of gotten over the whole thing, but now has an entire new family to enjoy, so I suppose it’s not altogether a disaster. In fact a couple of them have visited with our mother and I have enjoyed meeting them.
[HS]'s biological father died a few years ago. RIP.