Look, you didn’t know before and you aren’t certain that she isn’t related your father. Since that knowledge would in no way make things better as far as harmony in your family is concerned, then you should simply let things lay as they are. Even if money is involved.
There are probably many secrets which may be hidden in your family (like finding out who really is the favorite or who stole something that they shouldn’t have) and there’s not much point digging up the past. If that person wishes to know, they can get a DNA test on their own and determine if they are related to the rest of you by both parents, or just one.
No, I would not want to know. The only reason would be in a case scenario where the father who raised me was an absolute miserable jerk, who no one would want to call dad.
The only reason I can think of to investigate this is for some medical reason, like an organ transplant or a rare genetic disease. That doesn’t apply in most cases.
This happned to me or more precisely, my oldest son when an inlaw with a grudge against my son’s Mom decided he “had” to know. He was devasted. This was some years ago, and I may have mentioned it here. I still avoid this person at all cost as I’d like to do some things best left to dark alleys.
Other than the medical reasons Broomstick mentioned, I can see maybe wanting to go into it if the sibling in question was always treated noticeably worse than the other siblings by the supposed father. Then it might be useful to her to know that it wasn’t because she simply wasn’t worthy of her father’s love.
Otherwise, no, I don’t think I’d want to know. My father changed my nappies and walked me to sleep at night and played pretend games with me and put up with me when I was an obnoxious teenager and all the rest of the stuff that counts. He’s my father. Anything else - especially if we’re talking about dead people - is academic.