Was it wrong to deny who I was, or was it no big deal?

My mother started cheating with a man 10 years younger than her and subsequently left her husband (not my father) to be with the guy.

She didn't want the new guy to know her true age so she told him I was a "family friend" and not her daughter. She demanded other family members go along with the charade, and a couple of them refused, saying they wouldn't announce who I was, but they also wouldn't deny it.

She kept this up for at least 2 years, until lying became too complicated because they started living together and I relocated back to the same city. Eventually, she forced ME to tell him the truth, and in a weird way, twisted it to make it sound like it was my idea.

When I later told her that denying I was her daughter really hurt my feelings, she denied ever doing it, saying no, that never happened. And then she said "well, if I did do that, I don't see why it's such a big deal."

Was it a big deal or am I being "too sensitive" as she said?