In short: maybe. My grandmother gave me an old family Bible about a year ago. Like most old family Bibles, it has notes about genealogy in it, going back to the early 1800s, and contuniuing to the '90s, although the Bible itself is only from the early twentieth century. Included in the family history was a marriage of my mom to a man I had never heard of. Based on stories from my parents, the only way I could make the chronology work is if my mom cheated on her first husband with my dad for a while. My parents broke up for about a year, and then it seems when she separates from husband number one, they got back together. It was another nine years before I was born, so no doubt about my parentage, but I suppose it is remotely possible I have an older half-sibling. What really amazes me is the length the family has gone to to hide husband number one. I have never heard him mentioned by anyone, and I'm not sure my dad even knows about him. As far as how this has affected me, it helps that it wasn't like most other cheating stories. But I do see my mom differently for sure, and I'm much more cautious about my extended family knowing they've kept a whole marriage a secret for so long. If I found out my dad knew, and for sure that my mom was cheating, I would definitely have to re-evakuate my moral upbringing, and it would likey drive a wedge between my parents and me.
If your family's reasons for hiding the whole event are similar to my dad's, it'd be to preserve the harmony between your familial unit. It's a lot easier to think Mom and Dad have just been the perfect match since day 1 instead of the rocky beginning they took time into covering up.