If your mother is still living, my suggestion to you would be to keep it to yourself....
My mother and father, and the other father, are all deceased. Just the other mother is still living.
I think the new half-sibling is trying to determine whether or not to ask their mother about this.
“.....If your mother is still living, my suggestion to you would be to keep it to yourself....”
My suggestion would be to get her to take the 23&Me DNA test, but not tell her what you know. Then I would explain to her that you really want to have documented any medical DNA conditions that might be inherited by you or your children her grandchildren as to why you want her to test.
That will help everyone who really wants to know more to be able to sort things out in the future.