That's a really good thought. I never considered that Anakin might be the other. I think in all likelihood this is probably imbuing the line with more meaning than was originally intended, but there's nothing wrong with that. Happy accidents and all. Like the way Obi Wan reacts when Luke asks how his father died, originally the hesitation and look were meant to convey that Obi Wan was trepidatious about having to reveal the details of his death to Luke. Now it appears like Obi Wan is considering how to tell Luke without revealing to him that his father became a genocidal monster. When ESB was written Leia wasn't Luke's sister and wasn't going to be. According to Gary Kurtz they had instead envisioned a sibling that had been hidden outside the galaxy. It wasn't until Jedi was written that the Luke Leia connection came in to being and that line in ESB took on that possible implication.