Hi, bellemere, I met you in poetry, and if I ever finish translating that rather difficult Spanish poem you posted (volveran las oscuras golondrinas) to my satisfaction, I'll go back and post it.
We read "The Thirteenth Tale" on the old SeniorNet, and if you repeat your question in The Library, you will probably get a bunch of answers. It's been a while, so all the details have faded, but I'll comment on twins, since I am one. You know my twin, she's JoanK, who posts a lot in poetry.
It seems to be common to long for a twin as an "other self", someone who would totally understand you as no one else could. That's an understandable longing, and the book is full of this feeling, but I always felt it was unrealistic. Joan and I are very close, but more like close sisters. You can't really have an "other self". But during the course of reading and discussing the book, I saw that I did have a kind of fierce feeling about twinness, and I'll never know if this is stronger than loving sisterhood, since I don't know what it's like not to be a twin.
I'm too tired and sleepy to say more now, but ask your question in The Library, and I'll join in.