IMO if you weren't raised together, didn't live together and you have no blood relation...then I wouldn't counseled them a sibling. But that's just my opinion.
I only put the 2 sisters whom I grew up with and considered my "sisters" even though we are all actually half to each other. I have several step siblings that i am close with now as an adult but thought it would be too complicated to write "6" and have to go in to detail. Regardless, I don't think it really matters. It's never been mentioned.