The difference, by the numbers
They have no ingredient in common: the Between the Sheets has white rum, Cognac, triple sec and lemon juice, the Grand Margarita tequila, Grand Marnier and lime juice. In parts, a Between the Sheets is 1 rum, 1 brandy, 1 triple sec and ¾ lemon; a Grand Margarita is 3 tequila, 2 Grand Marnier and 1 lime. By volume, the Between the Sheets is 55% spirit, 27% liqueur and 18% citrus; the Grand Margarita is 50% spirit, 33% liqueur and 17% citrus. Different bottles, similar balance: that makes them close cousins on the ratio map.
They are about equally strong in the glass (18.7% and 19.0% ABV), but the Between the Sheets is the bigger drink, so it holds more alcohol (2.03 standard drinks against 1.69). The Between the Sheets has more calories (about 233 against 200), with about the same sugar (8.0 g and 7.8 g). Both are shaken; the Between the Sheets is served in a coupe, the Grand Margarita in a rocks glass.