The term Mom and Pop Sith is strictly a matter of modern usage. It was coined during the era of rising popularity of space travel, as a way to combat the trend of galactic Empires. The term was meant to stress on the positive points of small local Sith communities, especially the particularly family friendly aspect of their violent acts of destruction.
The term 'Mom' or 'Pop' was used by the Master Sith as per gender, while the apprentice adopted the spousal role regardless of gender or sexuality. This engendered resentment among many apprentices, especially when called upon to fulfill certain spousal obligations more thoroughly. This may be one of the origins of the trend of killing one's master.
In their hey-day, Mom and Pop Sith organizations enjoyed total monopoly over Evil on a planet. They were a convenient place for people to go for Evil schemes, contract killings and totalitarian mass subjugation as needed.
Then space travel was invented and suddenly they found their strangle-hold on local violence threatened, by certain larger organizations, like the Galactic Empire, who were more powerful and had more resources. This era witnessed the PWNage of many Mom and Pop Sith under the might of these conglomerates.
However many Mom and Pop Sith managed to hold their ground for a while. They began to stress on the positives of small local groups. sure a Galactic Empire was big and powerful and had droids and clones and ships and could wreak havoc on a scale heretofore unimaginable. But Mom and Pop Sith were all about the quality of their service, and you knew that when they killed you and tortured your family, it all very personal. Your death was the result of true hatred, not the collateral damage of a cold lumbering force with bigger fish to fry.
This helped Mom an Pop Sith hold out a little longer, with support in the form of a grass-roots movement started by Hippies. This support however was largely due to the fact that they wanted to see the Sith divided and weak, making it easier for their own Multi-Stellar Corporation to survive.
Ultimately, however, the main advantage of being a Multi-Stellar Corporation was something a local planetary group could not compete with (a few did try, but this actually destroyed any business prospects they ever had completely. duh.) and Mom and Pop Sith organizations have been almost completely wiped off the face of the Galaxy. For now....