You understand absolutely nothing of what was happening. Because you weren't there and because you're 16 years old.

For one thing, there's no reason to assume your parents were having an "affair". There are a lot of couples who are split up informally who don't bother to finalize their divorces until later. I realize that at your age, technically correct is the best kind of correct, but you really don't know what their lives were like, at all, at the time of the beginning of their relationship. If they were living on their own, separate apartments from their separated spouse, would you really argue that they couldn't date? Many people do exactly that and only finalize a divorce later. There are states where you had to be "legally separated" for three years or longer before entering divorce proceedings — and in that time, a lot of people moved on with their lives.

And the fact that your sister grew up without your father — again, you have no idea what happened there. It could be that — and has happened many a time — that abuse and neglect allegations were being made against your father, without a grounding in reality, and his access to his daughter was restricted. Or that your half-sister's mother wouldn't agree to visitation. Or that your half-sister's mother hated her daughter's father so much that she was constantly shit talking her ex to her child.

Your parents have been living in a stable, loving family for 16 years. You have the right to ask some questions, but you have to stay the hell out of the assumptions.

It may be that his estranged daughter is such a painful topic that he didn't want to bring it up over the years. It can't be easy.

Act with some compassion, kid. Stop assuming that your parents were having a soap opera-style life. Ask questions.