When Anakin gets himself blown up - or whatever really did happen to him - it doesn't matter that there hadn't been drop of Lars blood in his veins. Owen would sooner chop off his right arm than abandon his brother's son to those damned wizards.
Maybe Luke is always his brother's son. Anakin casts a long shadow, even from the grave. Besides, Beru and Owen never wanted children. They didn't want Luke, and they never thought of adopting him. But they've risked their lives and given up their youth to do their duty by him, as his aunt and uncle, and they've done their best. They brought him into their home. They've kept him safe for over fifteen years. Nobody, least of all Luke, doubts that they love him.
He's not their son: not by blood, not by adoption. But people love their nephews, too.