Weston Theater Company concludes its 89th anniversary season with “The Twelfth Night Show,” a brand new musical very loosely based upon Shakespeare’s “Twelfth Night.” Somehow, it captured the gist of Shakespeare’s plot with laughter, unexpected poignancy and not one “forsooth.” Last Friday’s preview performance by four talented actor-musicians delighted an audience eager to set cares aside for awhile for a convoluted tale of theater, perseverance and love. It was all very silly fun.