Again. We either have records. Or the apparent words of both your and our ancestors. Between our lack of origin records and your hangovers/ intentional lack of written confirmation of anything. It's a miracle if we don't find out we own the fang.
The blood Raven's don't actively steal from other chapters. From Chaos yes, and xeno's, the odd relic that was just left lying around with no name plate or heraldry attached. Mostly chaos.