The way Alexander said that he loved Tess though was pretty intense and didn't seem brotherly to me...as I said I've had a creepy feeling regarding their relationship since he was discovered. However I think it will turn out that Tess is a secret Luthor sibling and that Lionel's appearance (I don't think it was just limited to that flashback scene) will be some kind of uncovered tape or something explaining Tess's patronage, perhaps she will eventually be used to stabilize Lex, and possibly cause her death. After all, Tess never appeared in Clark's dream did she? 
