I think when people are afraid of " airing the dirty laundry" of the family it is because they can't face the truth themselves. Your mom probably knows what happened to you because you brought it to her attention once before and she shrugged it off.Now she doesn't want to have to admit it.She may feel guilty for not doing something about it then.
If you are working on healing, then she might find herself being held accountable for not protecting you, and that part probably scares her.

My mom still wants to believe my father was perfect. .he is dead too.. and she does all she can to deflect any memories I might try to talk to her about. I think it is so she can avoid taking responsibility for her role in the abuse.

People that want to keep family secrets have something to hide.Dirty laundry and family secrets are the same thing aren't they?No mom wants to have to admit that she failed to protect her child from a predator.

I think you have to focus on yourself and your own healing process. Do whatever you must do to facilitate that. And if you are beginning to build a relationship with your mom, then be honest with her and tell her again what happened to you and how you felt when she ignored you then.If she doesn't believe you.. so what.. you know the truth.

I think you have to build a relationship being as authentic as possible, so telling her how you feel, even if only in a letter, is important.

I hope things work out for you.. stay strong.. you can find healing.. and peace...