A MIGHTYWOMAN isn’t quite the right title for this hour of angst, as Three (Half-)Sisters kvetch about the deterioration of their common mother on her funeral day. The casting won’t convince you that they’re sisters, but the acting might. ALEXELLIOTT, who might be a young Julianne Moore, never makes a wrong move as the middle sister, right down to the last scene-stealing hair flip. The other two are OK, but not yet un-self-conscious, and everyone (including the audience) would have profited from more experienced staging in so confined a space.