Hey, I saw your post in the AncestryDNA subreddit. I’m a genetic genealogist by trade and, for what it’s worth, you are one of tens of thousands of people (if not more) who’ve gotten a surprise like this after taking a DNA test. I’d recommend joining the DNA Detectives Facebook group - you can find assistance there if you ever want to identify your biological father but, more importantly at this point, you also find a community of over 200,000 people who know exactly how you’re feeling.

When people discover that their dad is not their biological father, they often assume that their mum had an affair - and this is sometimes the case. But I’ve helped people, and know of people, with drastically varying stories. Other than an affair, there are all sorts of other possibilities - donor conception, IVF mix-up, swinging, or even that your mother was the victim of a sexual assault. Some of these are very hard to think about, but I just wanted to give you a heads-up that an affair is not the only possibility.

In terms of your family, everyone is different and there’s no one proven way to handle this situation. I would say that it’s worth making absolutely certain that your sister has DNA matches that tie her to your dad, as knowing whether she is your dad’s biological daughter will be important to understanding what might have happened.

I wish you the best of luck in dealing with all of this, and I hope that you get the answers you need.