Who's the dad?

Alright when i was 20 my mom re-married a younger man that at time had 2 children. "sally" was 3years old and "bob" was 6.

For years i knew sally wasnt his actual daughter. His wife at time cheated and my stepdad. Stepped up and sign his name. Anyway his kids visit every other summer. Now Sally is 16 and Bob is 19.

Bob through years has always been little different and was diagnosed with a mental illness. But his mom pulled him out school and denies anything. My stepdad is not much help. He's tried many times to help him. But Bob denies his health issues. He's very smart and has very bad social skills and believes everything his mom says..

Anyway my step sister, Sally. Is very smart and is growing up waayy to fast. But she was always spoiled on her mother's side and "her dads side". Thats thing they both dont know. I, as a mother and hardly knew who my dad was. I feel she should know and Bob too.

So last year there mom got divorced and her ex told Bob. That Sally was actually his half sister and that she has different dad.

Bob calls his dad. Which i was there when this phone call went down. I can hear Bob mad and crying. My stepdad just lied. Well kind of lied.

"No, Bob. She's your sister and my daughter. I love both of you."

I understand the timing was wrong, but i just feel they should by now. For me if i was my stepdad. I would of said something while ago.

What you ladies think?

:Edit:

I know its not my business. But as a daughter/mother and care for them 2. i feel its about time she knows. She also lives in a small town and her real father lives few houses down. HOW i know is my stepdad told all of us and is concerned because the man lives down street from her mother. My stepsister and i are very close and i dont want her to be mad at me for knowing and she doesnt.