Status

A few years before I was born, my mother had an adulterous affair and got pregnant. My father knew about it and was angry at first, but then he forgave my mother and remained married to her and raised my brother as his own child. My parents kept secret from us for many years that this happened and hid the fact that my older brother is not my full brother. Only after my brother and I grew up did we learn the truth.
Presumably a child born from adultery is a mamzer, which would make my brother a mamzer. But I also understand that once a woman has been unfaithful to her husband, she is forbidden to him and they must divorce. Well, my parents did not divorce and have married to each other for almost forty years.
My question is, since my mother was already forbidden to my father at the time of my conception, and he knew that but remained in his marriage to her, does that make me a mamzer?