Severus Snape wasn't the only Snape born on that night. He has a ten minutes older Brother Seneca. [Severus is an old Roman word for stern, while Seneca means old] Petunia didn't and as Dursley but as Snape and Harry would newer encounter Mr Dursley...
As children, the Snape brothers couldn't be more different. While Severus was quiet and closed Seneca was loud and on-going. At least as much as you could get around Tobias Snape. You know the two tips of twins? One is identical and the other isn't? Seneca had darker skin, his hair was wavy instead of grassy, his nose was smaller and he had a more masculine body-build. Both brothers were close but Seneca obtained some friends in the neighbourhood and when his brother finally found the smaller Evans sister the older boy saw the way his brother was around her and didn't interact with her so she could be only his brother's.
When at home whit drunken Tobias Seneca became an anchor to his kin. He eider tended to his mother's and brother's wounds or made Tobias switch his target with Seneca. As the Hogwarts came to the lives of the Snape brothers Seneca went to Hufflepuff and Severus was put in the Slytherin. He still kept his distance from Lily but unlike Severus, he becomes a leader of first-year Puffs. He kept his brother close even when he started going dark and semi-accepted his friends and semi-joined them. At least when it came to learning of the Dark Arts. He hadn't joined the fight against the Marauders and his only interference was since the third year when he became a perfect and started to give both sides detentions.
When both boys came back for summer after the fourth year they found the mother in the critical condition. While Severus ran for help aka phone the police and ambulance from the neighbours Seneca went mental and nearly killed his drunken father with his bare hands. When the authorities came they placed the boys at Evans for a time being. Seneca wrote to his maternal grandparents and while waiting for Eleen to wake up from a coma he became close to Petunia. She started to accept her sister again and in a week's become attached to the boy. Ten days into the summer holidays Eleen Snape was transported to the St. Mungos and a wary angry couple visited the muggle police station to make sure that Tobias knew that he was a dead meat if he gets out. The Prince Matriarch and Patriarch then came for their grandsons to the Evens and took them in. The healers at St. Mungos were too late and Eleen passed away and her sons became wards of her parents.
When they came back to Hogwarts Seneca estranged from the Slytherins as he now had a muggle girlfriend and stopped being a ladies man. When the legendary kill-the-Snivillus incident happened the boy took it as an assault on his family. He and his Puff friends went hard against Sirius Black and as he for a time being didn't have the protection of the other Marauders he was landed in the hospital wing. Paradoxically Regulus Black took it as an insult and went after the boy only to end up in the hospital and Slytherin lost the first game of the year without their seeker as McKinnon caught the snitch. It was an advertisement that Snape-Prince brothers weren't a fair game. On the other hand it lad Severus and Seneca to have a major fallout. The incident with calling Lily a mud-blood was the last straw for the older Snape and since that he left his brother to fend for himself.
In the next two years, much hadn't changed and the brothers graduated. While Severus took a Potion Master apprenticeship his brother took an Auror training and a half a year after school married Petunia. No more hard filings were there between the Evans sisters anymore and Lily came to the wedding with her boyfriend James Potter. Seneca and James became acquittance if not friends. On the Potter's wedding, Petunia came already seven months along and soon after Michael Seneca Snape was born.
As the times became more dangerous Seneca took his family into hiding. Now he was a fully trained Auror under Moody. Unlike his mentor and most of Aurors he had no regrets and used the Dark Arts against the Death Eaters so they went after him.
He made a peace with the Marauders. At that time the Prophecy was made. There were three possible candidates to be The Child. Longbottons and Potters went to hiding and stressed out Severus came to inform his brother of the danger as soon as he found out that he was going to be an uncle for the second time. The brothers hadn't seen each other since Hogwarts and Petunia still hated her brother-in-law.
Lily mentioned the Fidelius charm to her sister and brother-in-law. They liked the idea and she became their secret-keeper. Then at that Day in summer Neville Frank Longbottom, Harry James Potter and Magdalene Petunia Snape were born.