Thank you for bringing this question to Just Answer.
Even though this incident happened 60 years ago, I doubt (but no one could say for sure) that this would be an incident one would forget. That said, if it were a one time occurrence, a person could certainly, out of shame, push it out of their memory. Remember that 60 years ago, sexual abuse was not the openly discussed issue it is today. That isn't to say it didn't happen, because it certainly did, but that the "label" didn't exist. Many men who perpetrated before sexual abuse was defined and publicized call what they did "fooling around" or "just touching" and most of them felt at the time it (and it pains me to even type this) that what happened was "no big deal." Given that attitude, the time period, and the lack of significance it was given back then, it would be possible for a man to convince himself he did nothing wrong.
Without a clear memory, I would suggest the woman go into counseling. She needs help dealing with the feelings of violation and lack of trust that can arise from the experience of being molested.
Rather than try to get the brother to admit to something, the energy should be put into helping this woman heal. It is possible that the person who actually did this is someone else--very often the victim cannot bear the pain of remembering that a parent or grandparent was the perpetrator. This is a minefield that needs to be dealt with in therapy.
The method I have found helpful in working with people who have a strong feeling that "something bad happened when I was young" is EMDR. The true damage of abuse and neglect is the distorted, negative self-image the victim is left with. EMDR (eye movement desensitization & reprocessing) can help correct the victim's self-image, and takes the painful emotions out of the memories. Here is a link to a video that gives an explanation and demonstration of the technique:
CBS video report on EMDR
Without a clear memory, it will only be destructive to continue trying to figure out who might have done this. Sometimes the victim supresses the memory of who did this to her because that memory is too painful--someone who she should have been able to trust completely. This especially happens in families where the father or grandfather is thought to be "a very good man who would never do such a thing."
Most therapists who deal with adults with long ago abuse put the focus on the effect the abuse has had, rather than on who did it (especially when prosecution isn't possible). While the idea of an apology is appealing, an apology alone will not heal the damage that was done. If she is open to working on this issue, you could find an EMDR therapist in your area through these links:
Therapists trained in EMDR
I hope she makes the decision to heal this trauma so you can get out of the middle of this situation.