That is hard for you OP . I think if there are people in the family that your father may have potentially abused when they were younger , you do need to do something about it .This may include your siblings and any grandchildren , cousins etc .
Whilst he is dead there is obviously no current risk but potentially others may still be suffering from abuse that could have taken place in the past .
The NSPCC are usually a good source of advice , so you could check with them .
I think what can be really hard for those that have suffered abuse is the “ conspiracy of silence “. That is ; that others knew and either minimised it / brushed it under the carpet or went into denial mode .
It may be that nothing abusive happened after this and your father got a shock / felt guilty / worried about others finding out and chose other outlets eg porn , sexual relations with those older . You don’t know .
You can’t unknow - what you already know and I would try and share this burden with your siblings at the very least and discuss what if anything you say to your Mum .
You said he died earlier this year , so not sure how long ago . Grief isn’t linear so it may be that others are still in an active grieving mode .
I think you will also need to consider the timing if you do choose to tell your siblings . I would if you can try and do this in person , somewhere that you can have a bit of privacy and forewarn them that you have found out something troubling about your father .
You may not get the reaction from them that you expect , they may go into denial mode .
Firsts for a recent death can be very hard ie the first Christmas , wedding anniversary of your parents / birthdays - so also try and avoid / close to those dates .