Actually, the sibling did first appear in OB, in an epigraph, but they are invalidated from hint 4, I think
No, it is not
1. This character is humanoid, I think.
2. This character is a Splinter
3. This character has Investiture that is from the Shard of Odium, most likely.
4. This character has visited Alethela
5. This character first appeared in Oathbringer
6. This character can't move, I think
7. This character can Surgebind.
8. Shallan speaks to this character
9. This character has been influenced by an Unmade
10. This character was born from Honour and Cultivation
11. This character is an enlightened spren*
*I really thought this would've been obvious from 2, 9, and 10