AITJ for not allowing my biological daughter to tell my younger daughter that she is actually her mother?

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TL;DR: I am 60 years old. I have 2 daughters, although biologically one of them is actually my granddaughter.

About 20 years ago, my older daughter got pregnant at 18. The child's father did not want to participate in her life, and the daughter herself dreamed of entering university and did not want to become a mother. She seriously considered the possibility of giving up the child after birth.

I could not allow my granddaughter to end up in the foster care system. Therefore, I suggested another solution: I raise the child as my own daughter, and my daughter continues her life. She agreed. I adopted the granddaughter, and we agreed that this would remain between us. Others also knew the truth but respected our decision and never spoke about it to the child.

I was there for every step of her life. I didn't miss a single birthday, school concert, or doctor's visit. I sat by her bed when she was sick. I worked overtime to pay for her clubs and education. For me, she was not a granddaughter, but a daughter in every sense of the word.

Meanwhile, my older daughter moved to another city, graduated from university, built a career, got married. She took almost no part in raising the child.

They saw each other a few times a year, and my younger daughter always perceived her simply as an older sister. Although friendly relations did not develop between them.

But recently everything changed.

My older daughter started making hints to the child. For example, saying things like "you have no idea how close you and I are" or "someday I will tell you something important." I asked her to stop, but she doesn't listen.

After a long conversation, she confessed that she wants to reveal the truth.

And this is where the most painful part begins. She and her husband want children. Due to medical problems, they cannot have their own. Now my daughter speaks about wanting to take back her daughter and build a mother-and-child relationship with her. But how can you take back a child after 20 years?

She doesn't know her favorite books. Doesn't know that she is afraid of thunderstorms. Doesn't know her friends, dreams, or plans for the future. She wasn't there during any important moment of her life.

And now I am afraid that this whole story is not about love for the child, but about the desire to save her own marriage or give her husband what he always wanted, because then her financial well-being will increase.

I do not forbid them from seeing each other, but lately communication is limited. It seems to me that she is gradually preparing the ground to turn my younger daughter's life upside down.

I understand that the truth sooner or later may come out, but I also cannot get rid of the thought that a person who 20 years ago voluntarily gave up motherhood has no right to demand it back just because now it has become convenient for her.

AITJ for trying to protect my daughter from the situation and not wanting my older daughter to use her?