While she may not have been in love with him, I don't think it was impossible for her to have some messure of feelings for Serverus. After all, they were best friends until their fifth year. Her feelings, in my opinion, were not exactly romantic in nature. It was a love for her best friend, like Harry's love for Hermione.
J.K. Rowling has confirmed that Lily loved Severus as a friend, and she COULD have loved him as a boyfriend, but when he chose to be a Death Eater, he lost her.