Starlights story is a little changed in my nextgen, but only how she is treated / acts after Twilight 'befriends' her. I'll elaborate more later, but the short of it is she isn't as close to Twilight and her friends as she ends up being in canon, she never saved Equestria from the Changelings, and doesn't end up being the headmare of Twilights school, though she does end up working there.

Sometime after the changeling event, which again Starlight was not a part of, Starlight went off on her own, letting Twilight continue to do her thing and trying to learn more friendship lessons outside of ponyville. Not knowing entirely where to start she headed to The Crystal Empire to meet up with Sunburst, after staying with him for a short while she decided she may as well get together with him, because she figured she must love him if him leaving as a child had kept her so upset for so long. It wasn't hard to convince socially-awkward Sunny that it was the right and logical thing to do, so they started dating.

Starlight pretty much forced every new 'step' of their relationship, either subtly, or not so, mentioning things that Sunburst should do, like bring her flowers or take her on a date to that fancy Crystal Berry restaraunt. Starlight essentially went though their relationship like a list; dating and courting, romantic gestures on any milestones, practically forcing Sunbursts hoof into getting just the right ring she wanted and dropping a thousand hints about how he should propose to her, and still being upset when it wasn't 100% to her ideal plans, and the wedding that she had every say in, only letting Sunburst invite his mother and a few close ponies, but having no say on decor, music, or anything else.

For his part poor Sunburst was simply caught up in the tidal wave of Starlights abusive nerosis, doing what he could to please her and becoming more and more anxious of upsetting her as their 'relationship' continued, but poor Sunny couldn't change it, even the few times he would try to bring up to Starlight that maybe she had the wrong idea or that they don't need to date 'by the books' she would quickly assure him that she was in control (not very assuring really) and had everything planned out (again, not what Sunny wanted to hear), Starlight simply couldn't communicate with him, everything she did put him down and made him feel lesser, like she was always right and he was always wrong, and Starlight never quite saw her own actions, and when she did she easily conviced herself that she was right and he would be happier for it in the end.

Of course the next step after their marriage and quick honeymoon in canterlot would be to have children, so Starlight quickly got pregnant with her first foals, the twins. She noticed how having babies seemed to breathe new life into their relationship, Sunburst was caught up in a tidal wave again but this time is was being a father and having new foals to look after, and he was a good father! Sunburst would always check on the foals needs, reading every book he could, and playing with them in ways that helped build their motor and neulogical functions, he did afterall have experience being Flurrys babysitter and mentor.

Starlight did have her own ideas on parenting, but her take was much more rigid, she didn't appreciate the babies crying for 'unnecessary reasons' like needing attention or comfort, claiming feeding and changing, and danger, to be the only reason they should cry. Babies were simply far too illogical. She would also try mentoring her children, though her methods were harsh and tiring. The foals had two parents who were both very well read on rearing babies, but only one who had the heart to care for them, while the other was a harsh taskmaster, this would affect how the children would grow up and their personallities.

As the children would grow Starlight would notice cracks in the relationship between herself and Sunburst, for which she would try every trick in her manipulative book to solve, until she found one what worked, having another foal. Once again Sunburst would be swept up in the joys of his new budle and there would be no time for petty arguements or chances to disagree with Starlight and her methods, perfect in her eyes.

After Chaser came along Starlight tried using other tactics to keep Sunburst under her hoof rather than having another child, she couldn't keep doing that forever right?, but Sunburst would keep becoming disillusioned by her and her attitude, it didn't help that a certain blue-maned guard stallion offered him more comfort and reassurance than she ever did and was gently helping him to understand that he was in an abusive relationship, with Flash Sentry at his back Sunburst started to flourish again after years of wilting under Starlights strict regimes, Starlight would notice that she was losing Sunburst more than ever and took her last ditch effort, she would have another baby. But Sunburst had been using protection and she had to foil it with her own magic to force it, she tried to convince Sunburst that his spell must have failed but he knew better and had the spine to back himself up now, he denounced Starlight and got a divorce.

The divorce was messy, these things can't be entirely clean, and while Starlight tried to hold onto what she could she ended up with joint custody of Chaser while Sunburst got to take the baby for the majority of the time, the twins were too old for custody arguments, by this point and didn't care much, they would claim to have seen this coming for many years and Chaser somewhat agreed, the abusive nature of their parents relationship didn't miss them, they simply stayed out of it the best they could.

Sunburst kept the house in The Crystal Empire and tentatively started dating Flash Sentry shortly after the divorce. He named his baby Luster Dawn, in the hopes that his future days would bring better things.