At the end of Episode III, there are two new babies who are down two viable parents—mom's died of a broken heart (or something [1]) and dad has become a tyrannical right hand to the galaxy's freshly minted dictator. They need homes but, more importantly, they need to be kept safe from interfering influence. Most of the Jedi have been wiped out, and resistance to the Empire is a bad bet when it's on its way in; Palpatine has too many resources, he planned his takeover too carefully.
Yoda and Obi-Wan knew all this coming out of the Clone Wars. They knew that even with the handful of Jedi who survived the Purge at their disposal, they had no chance of taking on the new regime. It was better to let Palpatine get comfortable, to hide away and await a better opening. And considering the power that both Vader and the Emperor could claim mastery over, using Anakin's kids were a good bet; after all, their dad was literally conceived out of raw Force trail mix and some of Shmi Skywalker's genetics. They were bound to have similar power at their disposal, the better to knock off dear old dad with.
Here's the problem with children—they grow up to be fully realized human beings. The Jedi knew what they were doing, taking infants from their parents to indoctrinate them into the old Order. Initiating kids before they could talk, becoming their true family, resulted in “better” Jedi. (Translation: Jedi who do as they're told by the Jedi Council and their mentors.) But Anakin's children were better off being raised out of contact with Jedi. That way, if someone managed to locate Yoda or Obi-Wan, the Only Hopes of the galaxy remained safe and secret.
Via MyFaceGooglePlusSpaceBook, “This is Why Obi-Wan Lied to Luke Skywalker About His Father | Tor.com [2]”
It's a long article, but it does seem to explain why Obi-Wan outright lied to Luke about his parentage. I never expected the Good Guys to lie, but the Bad Guys? Yeah, I expect them to lie to get ahead [3], that's what Bad Guys do!
But not the Good Guys! The Good Guys don't manipulate people [4], don't lie [5], fight fair [6], and above all, never act like a vigilante [7].
Um … are we sure the Empire aren't the good guys? [8] Yeah, they're incompetent [9], but really? The Bad Guys?
[1] http://io9.com/maybe-luke-and-leias-mom-didnt-die-of-a-dumb-broken-
[2] http://www.tor.com/blogs/2015/01/this-is-why-obi-wan-lied-to-luke-
[4] https://www.youtube.com/watch?v=532j-186xEQ&t=30s
[5] https://www.youtube.com/watch?v=hFB16GCocfw&t=53s
[6] https://www.youtube.com/watch?v=Oh4l39Lo194