The Sidecar family of cocktails are similar to the Sour, but with the addition of a liqueur. This can add a lot more complexity to the flavor of a drink. But it also makes it more challenging to get the flavor balance right.
While the proportions in a Sour rarely change, the addition of a liqueur adds both sweetness and more alcohol to the mix, so things need to be adjusted. Typically this means a little less of the base spirit and a reduction or even removal of the syrup. And since every liqueur is different, drinks in this category tend to end up with a varied set of ratios.
This category is named after one of the earliest of such drinks, the sidecar, made with brandy, lemon, and orange liqueur. These days, however, the margarita is probably the best known drink in the Sidecar family — tequila and lime with orange liqueur added. My Tabs vs. Spaces is the same thing but with Green Chartreuse. And a White Lady is a gin and lemon with orange liqueur (and an egg white) added.
Credit for the six cocktail families goes to Cocktail Codex. I highly recommend it for an in-depth exploration of drink variations in each category