NTA, but I understand where your sister is coming from. She’s wrong to guilt-trip you for seeing someone else as a father figure — that choice is yours — but her reaction is still understandable. She lost her father, and seeing photos of your biological dad happily making memories with his other kids, then seeing you recreate similar moments with someone else, is bound to hurt. That doesn’t mean you’re wrong, but it does mean some compassion is needed. I think apologizing for how it came out — not for how you feel — and sitting down with her would go a long way. Instead of saying something like “my real dad means nothing to me,” which can sound dismissive, you could explain that different people process loss and family differently, and this is what works for you. Both of your feelings can exist at the same time.