I was a baby when my mom met my sister's dad. Something like 5 or 6 months old, not totally sure. He was straight up with her that I would not be important to him because I was someone else's daughter.
She was fine with that. Sounds like they moved in together within a few weeks of dating and he was never any kind of on with me. I was 4 when they had my sister Katie. They were married at this time too.
Katie was always the apple of her dad's eye. She and I had... a very off relationship. Sometimes we got along kind of okay, but I will pull back when she would start praising her dad and telling me how lucky *we* were to have him as *our* dad.
Even after her dad told her I was not his kid and she was the only person in the world who mattered to him, she would still bring him up to me as being amazing to us. Our mom was often around to tell me I needed to accept that I had a different dad to Katie and that needed to be okay.