Harry Potter and the deathly hallows mysteries?

Well we already know slytherin's locket is in the house of Black, it's mentioned that they find a silver locket when tidying that they cannot open. But I have a question....why is it that Harry doesn't see the thestralls until the order of the phoenix, when he saw his mother die? Does that mean Lily Potter isn't dead at all? Also, do you think RAB is Remus Black? And does anyone else think Dumbledore's brother Aberforth might make an appearance? Do you think the sorting hat might be a horcrux?