You could dismiss this swankily shot Latin American trifle as an upscale soap opera, but that would be an insult to soap operas. La Mujer de Mi Hermano is really two ''racy'' women's-magazine articles yoked into feature form. The first (''I Slept With My Husband's Brother'') sounds kinkier than it is: The husband (Christian Meier) is a dullard, the wife (Bárbara Mori) a vacuous beauty, the brother (Manolo Cardona) a cliché lout. The second (''Is Your Husband Gay?'') turns into camp when you learn the key character revelation is said husband's habit of getting his back waxed.