So my father apparently conceived my half sister on only one time, as a 50 year old man who was developing rheumatoid arthritis. Is this likely? Is it unlikely enough that I can basically consider it to be false?

Also, as an example of how weird this is, I remember my mother told me that she had told my half sisters mother that my father regretted having not spent more attention on me because I am like my father and he could have helped me but my sister is nothing like him.

The only subreddit I could think to put this in was this one, although this will have a clear bias. It didn't seem right to post this in a forum where people are trying to conceive. I would welcome redirection to a resource where I can get a medically informed opinion on this.

She is controlled by her mother and will not take a genetic test.