Commentary Sometimes, when I find myself thinking once again about my sister Joyce Jackson and the role that she played in the disability rights movement of the seventies, I’ll pull up YouTube on my iPhone, click on the documentary “The Power of 504,” and relive those electrifying days when this nation was captivated by the sight of young protestors in wheelchairs and on crutches—disability rights activists demonstrating in San Francisco and Los Angeles, New York City and Chicago and Denver. Feeling nostalgic, I’ll watch attentively—over the course of some 18 minutes—eager to spot my baby sister, to glimpse the ever-so-fleeting, grainy images of her lovely face, her undulating gait, her proud carriage. My sweet sister, who broke my heart one day decades earlier, when she muttered: “I’m black and I’m disabled. Who wants to marry me?” And suddenly there she is: at timecode 2:42 in the video, perched on a …