My sister had no idea that I would immediately know who this family and half-sibling were, due to where I live, and having had contact with the family in the interim years due to strange coincidences.
Then also, having found out that the person lived right near me, kind of gives me the feeling of God putting people in the same place at the same time, in an effort to be sure they meet up. (I have had several experiences in my life like this, which have been positive, but kind of eerie.)
It is up to the new half-sibling about whether to move forward now, it seems. They have a great deal more of new info to absorb, than we do, as well as to decide about asking the mother. Also about telling their own siblings, children, etc.
I would always advise keeping this kind of news to yourself until the unfaithful and his/her spouse has past on.
It is most likely that relationships will suffer. I would advise this half sibling of yours not to reveal this to his/her half siblings. It isn’t worth the chance that their relationship with their parent would suffer.
Grandchildren certainly do not need to know this kind of thing.
Honor your father and your mother. I think keeping their secrets falls in to that huge and very important commandment.
Your sister took it upon herself to make contact with this half-sibling without informing the rest of you?
Seems rather thoughtless.
At least you would think that she would want to know what the rest of you knew about this development.
I have several stories of incidents when I was face to face with two half siblings and a cousin without knowing who they were. It is indeed eerie.