It's hard. You weren't wrong. I think it is important for people to understand the difference between whole, half, and step siblings just for biological and informational purposes. I know some adults that don't seem to understand the difference which I find ridiculous. At the same time, they should understand that siblings are siblings PERIOD.
I have no full siblings but have 4 half siblings of which I am the oldest (3 I grew up knowing as my siblings, the other was a surprise we found out about more recently....Dad didn't even know). I also have two cousins that I lived in the same house with growing up. I also have one step-sister who was a teen when she joined the family and I was grown (I haven't been around her much especially since she moved away) so I don't really feel like her sibling (if we were raised together it would be different altoghter).
The sister I grew knowing as my youngest sibling was a bit puzzled when she had to do a family tree for school though. She always knew I was her sister and we had the same father but she said "since L. is my step sister, where do I put her on the family tree?". My dad said "She's your sister you put her under me". He didn't explain "half" which would have made things easier in the end.