I would tell her.If it was me I would blow the whole situation right up and drag it into the open .
This is based on a particular experience.
A friend of mine at University was the child in similar circumstances. Youngest of four boys, he was the child of a long-term affair.
When he was a teenager, his mother and “father” got divorced and his mother remarried quickly. To the affair partner.
My friend had always struggled with feeling different,that he was different to his brothers and that his “father” treated him differently.
His mother and father (who he thought was his step father) told the truth when he moved out of home to go to University.
He was sweet, sensitive lovely boy who helped me through one of the most difficult times of my life in a kind, intuitive, selfless way.
He got more and more troubled as he realised his life had basically been a lie. And that basically many people
in his life had known but no-one had told him.
He really struggled with trust. He felt betrayed. He didn’t understand why no-one (grandparents, family friends etc) had told him.
He just scraped through university and by the time he left he had a really bad drinking problem. He found it very difficult to maintain friendships and relationships.
We managed, to what extent we did, because my family background was fucked up to a similar level though in a different way.
He was such an amazing person with a really great soul and so much potential.
He spoke to me so much about things like wishing he had had a chance to live his life in the truth. Because that way he would be able to trust what people said to him.
He just got more and more isolated and withdrawn.