When I do DNA presentations or talk to individuals, I am careful to warn they may have unexpected results. It can be a Non-Parent(s) Expected (NPE) or something like an unknown family member showing up. I warn them that not all family members are going to accept them into the family. There are generally at least three sides minimum to these cases – side A, side B, possibly other sides, and the truth. The truth often is a mix of the various sides’ versions. At other times, the truth is none of the above sides.
I see comments in social media groups where a person assumes the new family will welcome them with open arms and hearts. That happens occasionally, but it’s less common than many people realize. In other cases, a family member who knows the truth will discourage DNA testing as they think they know some or all of the truth. People have a bad habit of covering up the truth. Back before DNA testing became so popular, it was easy enough to hide the truth. These days, many of these secrets are coming to light.
I will use myself as an example. There were questions as to who my Dad was. A sibling would never DNA test as they felt it would dishonor Dad. Other siblings wouldn’t test for other reasons, mostly due to potential law enforcement uses of DNA. A niece went ahead and DNA tested because she was curious about ethnicity. She showed up as a full niece, proving Dad was my Dad. He died decades ago so I couldn’t share it with him. Another niece also DNA tested and showed as a full niece. At that close a relationship, that’s proof of the biological relationship with Dad.
There are people who claim DNA isn’t proof. At more distant levels, 3rd cousin or similar levels, that can be true. At parent/child or half-sibling levels, even something along grandparent/grandchild, aunt/uncle, niece/nephew, it’s fairly reliable. The exception would be identical twins as the DNA tests generally only look at several hundred thousand markers and not the full DNA. There are minor differences a full DNA test could show where you could know which identical was the parent. In the same way, it used to be if an identical twin committed a crime, the DNA testing wasn’t detailed enough to know which one committed the crime. These days, they would look for the minor differences in DNA to rule in or out a specific individual.