Theory: Yang has a faunus fraternal twin.

Justification:

- Raven and Qrow are siblings in the same year at Beacon.  Unless we have reason to believe one of them was admitted to Beacon extra early (like Ruby), or their birthdays are essentially exactly 9 months apart, it's safe to presume they are twins.

- Twins tends to have a genetic component where a twin is more likely to have twins for children (i.e., as a twin, it is more likely than average that Raven would have twins).

- Holding to previous wild speculation, Raven is an upper-up in the White Fang, and a faunus.  

- Per voice of God, if a faunus and a human have a child, their child will either be a faunus or a human (presume 50/50 chance).  If Qrow and Raven's parents are a human and a faunus, they could conceivably have a set of fraternal twins who are split between the species as well.  Similarly, Taiyang is human, so if Raven is a faunus their kids would have 50/50 odds as well.

- We know that Raven took off shortly after Yang was born and that Yang always wonders why she was "left." What if Yang was actually a twin, and Raven left her behind while taking a faunus sibling with her?