What would you do if you heard your father sexually abused your brother when he was a child?

I have three brothers. Brother (1) just told me that brother (2) alleges that our father abused him as a child. He made the claims about five years ago and I am only being told now. There is a seven year gap between me and my nearest sibling. This poor brother (2) is now in his forties and is depressed and very quiet in himself. He is suffering from alcohol addiction. Brother (1) has no recollection of anything untoward happening but apparently for one reason or another brother (3) believes this did happen. I know that my father used to be a tyrant when we were small but could he have been capable of this disgusting behaviour? I do not remember him doing anything of this kind. He works for a charity and is very well respected in the wider community. He has totally mellowed over the years and to my knowledge is as lovely and helpful a man as you could ever hope to meet. My mother is a lady to her toes but was often fearful of my father in the early years. Could this really go on in your house and you not realise it is happening? Why was this hidden from me? Surely if it was true I should have been protected? Who should I ask? At the moment my mother is very ill with cancer so it would not be a topic for the Sunday lunch table. I know that this would finish her. As a family we deal with problems by digging a hole and burying them from memory. We have very poor links of communication with each other. I just don't know what to do. I do not want to visit or speak with my parents. I do not want to bring my daughter to see them. I cannot ask brother (2) if this happened him as he is in a dark place these days as it is and do not want to make things worse. Brother (1) regrets ever telling me and swore me to secrecy and my mother would just keel over if she heard. I feel my relationship with brother (3) is not good enough to just call him up and ask him about this either. What would you do? My head is all over the place.