Disclaimer: I don't own this. You are legally blind if you think so.
Charlus Potter married Walburga Back because he lost a bet to Pollux Black. Pollux had a troublesome time trying to dispose of his daughter Walburga. To be blunt, in the bridal marketplace Walburga had less worth than a bluestocking Victorian times because she was already mad a despite her young age and was the worst type of shrew. Things for Walburga looked so bleak that her father was willing to dump her on one her dimwitted cousins as last resort. Thus the only thing Walburga had to offer other than a bounty of gallons as a dowry was her fertility.
Unfortunately for Charlus, he could only shut up Walburga with sex. Unfortunately for Walburga, Charlus was an absent minded sort. So the Potters ended up with ten children until Walburga wised up and made Charlus a manless man with a well-placed dark curse during the birth of the eleventh child. (Yes, you read that right eleven children, more than the Weasleys.) In order the younger Potters were Caroline, Elizabeth, Beatrice, Sarah, Maragret, Euegenie, Cecily, Violante, Katherine, Maud and Leopold. It helped that Walburga had two sets of twins in one go but not really if you asked the opinion of one Severus Snape.
Instead of dealing with James Potter and Sirius Black and their back up crew, Severus Snape had to deal with Eugenie and her twin Cecily and Violante and her twin Katherine in Gryffindor And boy were the Potter girls vicious to Snape. In hindsight, if Snape was willing to share Lily as friend or even drop her as friend none of this would have happened. It wasn't too bad for Snape, he had a true in Slytherin with Leopold despite their age difference because they had a mutual enemy. And Maud was just air because she was a Hufflepuff.
So the younger Potters grew up understanding that sex was used for revenge, silence, ultimate submission and totally victory over the enemy. So when Snape ended being ambushed and tied up by the Potter Four in the Room of Requirement during their seventh year he ended up being up being the girls' breeding stock.
In the end, Leopold finally got the older brother he wanted. Severus Snape was forcibly married to the four Potter girls when they proved to be as absentminded as their father. Charlus Potter was forced to help Snape become a man that could support all grandbabies. Thus, Snape was too busy changing multiple diapers to join Lord Voldemort. Without Snape's potion expertise Lord Voldemort wasn't able to rise in power. However, Walburga still scorched her children off the tapestry on principle despite the cuteness of her grandchildren.