I grew up knowing... if I hadn't, I would have figured it out. It's not real hard to distinguish which one of my a-parents children is their (only) biological child -- she looks, acts, and even thinks just like them.
We (the other three who are adopted) don't -- not even slightly. I am not the first adopted in my a-family, so my adoption was 'no big deal'... it was just a matter of fact.
I'm sick to my stomach that your family member has gone 20 years living a lie. Even more so that she now has a child who she has passed a 'genetic surprise grab bag' to and a husband who, in all reality, could be her biological relative. UGH!
What a mess! I don't know your relationship to your adopted "family member" but if you are one of her adoptive parents, then SHAME ON YOU... SHAME ON THEM, if you are not one of them.
If you are NOT one of her a-parents, I'm revolted by the knowledge that other family members know and she has not been told. It's so, so very wrong.
Tell her immediately. If you are one of her a-parents, expect to lose her trust -- possibly forever. Expect her unbridled anger (which you/they deserve) and DON'T expect her to ever forgive you/them. Keeping HER OWN life and truth from her is unforgivable.
Tell her today -- right NOW. At least give her the opportunity to search for them and possibly know the reality of her beginnings and genetic/health history. At least give her the chance to confront her adoptive parents before they die (if they haven't already). At least give her the truth. She deserved, and deserves, so much more than that but that may be all you have to give her -- GIVE IT... NOW.
For the record, the content and style of your question leads me to think you are not one of her a-parents. I'm most disgusted by them, because that was/is THEIR responsibility first and foremost. I still don't like that others in the family know and nobody has told her... that is so unkind and so wrong. She will likely want to know exactly who knew and exactly how long they knew without telling her -- that is if she speaks to anyone in the family after finding out. If you want to have even a chance of maintaining a relationship with her, be the one who has the courage to give her the truth. Be the one who decides that this GROWN WOMAN in your family should no longer be lied to as a child and kept in the dark as a second-class citizen.
That is... if you care about a relationship with her. I would guess that you do as you bothered to ask. Do the right thing... Immediately.
p.s. For your own knowledge, please 'google' the term "late discovery adoptee" and read up on how many of them feel about it and how she may likely feel about it and/or deal with it.
Reunited adult adoptee and social worker who is appalled at how many "late discovery adoptees" there STILL are in the technology age.