And there are reasons for not contacting your half sister while the father you share is alive. It would let her know that he hadn’t been forthcoming with her, which might seriously disrupt a relationship you know nothing about. You can think that your father should have told his daughter and her mother about your existence — and think, too, that it isn’t up to you to decide this for him.

Note that these reasons for discretion may be counterbalanced by other considerations and, in any case, wouldn’t survive his death. He would have lived his life on the terms he requested. He made no covenant with eternity. The daughter might well be angry about his having kept this secret, but she might also take consolation in the discovery of a new half sibling. (If she didn’t, she would only have to tell you she didn’t want a relationship.) Whatever reputational harm might befall your late father at this point doesn’t fall under the term “unkindness.”

But you also envisage a situation in which your half sister discovers your relationship via genetic and genealogical databases. And here’s where things get complicated. You say that in order to preserve your father’s secret, you’ve avoided genetic tests that you otherwise would have wanted to take; you wonder about reconsidering that policy. The fact is that you were always within your rights to take those genetic tests and let the chips fall where they may.

How is this consistent with the value of respecting your father’s decision? There’s a principle, usually dated to Thomas Aquinas, known as “double effect.” The idea is that accepting a consequence that is the byproduct of an action is different from pursuing an action intended to bring about that consequence. It’s why the Catholic Church permits medical procedures that are intended to save the life of a pregnant woman even if it ends the viability of the fetus (by removing a cancerous uterus, say). Your right to use one of the services you mention isn’t defeated by the fact that doing so might, as a byproduct, expose your paternity to others.

When your father chose a closed adoption, he did not intend to prevent you from availing yourself of genetic and genealogical services he couldn’t then have imagined, and he would not have had the right to do so even if he could. (Sometimes the children of a closed adoption never learn that they were adopted, and so they have no reason to think twice about being genotyped. It’s a contingent fact that you do know.) By not availing yourself of tests that you wanted for other reasons, you went beyond compliance: Rather than passively respecting his agreement, you were, in effect, joining your father’s efforts at concealment.

It’s worth observing that not everyone will be on one of these databases, so there’s no guarantee that your half sister will find you this way. But suppose she could. Going on one of these websites now would have one advantage: It might allow her, if she was on the site, to decide for herself whether she was interested in unknown close genetic relatives. The decision to contact you would be left to her. Of course, her first probable response would be to talk to your birth father about the existence of someone who looked like a half sibling, but that’s not on you. Your birth father’s hopes for maintaining his secret must be recalibrated to match a changed reality. In an era of mass-market genomics, it’s hard to hide the twisting branches of our family trees.