There's no reason to assume that Shmi wasn't force attuned, is there? I mean, Yoda states that if Luke were to fail against the Emperor, there was another (presumably Leia) and she had no idea of her potential. Maybe some people carry the potential without it ever being realised? Or maybe by the time they're found, they're too old to start the training.
So, maybe the force does follow a family line (the Skywalker line) and maybe Shmi is sold in/by EPII by Watto to a group of people who, having heard of the exploits and repute of Anakin, have tracked her down and wish to 'experiment' on her to see if that line can be tempered.
My guess is that Shmi is bought from Watto (who, let's face it, needs the money now) by a group of investigators working on the Clone Armies who want to see if they can assertain from her why Anakin is so powerful and then genetically engineer Anakin's potential / power for their own troops. In the process Shmi is killed or assassinated by rivals who see threat of these investigations. On learning of his mothers death, Anakin investigates, uncovering a ghoulish sounding plot to build Clone Warriors, crusades against it and, low and behold, finds that the Jedi Council have been secretly funding / supporting the venture in recognition of the rising threat from Droid armies being build by Sideous.
Anakin's reaction is obvious - it was Jedi-Cloning that lead to his mothers death, the Jedi who supported the Cloning, so he's none too pleased. Instant reason to join the Dark Side, perhaps?
[ July 24, 2001: Message edited by: darthyogi ]