Ever since Twilight Sparkle first traveled to Canterlot High, she had a crush on the Flash Sentry of that world. However, she knew that a relationship with him could never work since they were from different universes. That’s when she realized there was a Flash Sentry right in Equestria. So, she decided to get to know him and see where that took her.  
One day when visiting the Crystal Empire, Twilight decided to strike up a conversation with (pony) Flash Sentry. After a casual yet enjoyable conversation with him, Twilight realized that this Flash Sentry was even better than his human counterpart. He was smart, funny, and generally had more in common with her than human Flash (who, she admitted, was just an immature teenager) ever could.  
Twilight and Flash’s friendship continued as Twilight spent more time at the Crystal Empire and they got to know each other more. After a couple years, their friendship even grew into a romance that went deeper than Twilight’s crush on human Flash. Soon, their little visits turned into dates as their relationship became official.  
After a few more years passed, Flash decided that he wanted to make Twilight his wife. After buttering up her brother and asking for his sister’s hoof, Flash took Twilight out on a romantic night on the town and proposed to her. Of course, she said yes.  
Twilight and Flash had a romantic wedding in Canterlot, free of any world-threatening dangers. Shining Armor reassigned Flash as Princess Twilight’s personal guard (and husband, of course), which allowed the newlyweds to move to Ponyville together.  
As the years went by, Twilight and Flash had two children together: Dusk Star and Galaxy Guard. With each child’s birth, Twilight and Flash breathed a sigh of relief when they discovered they weren’t alicorns.  
However, the infant years weren’t easy. Dusk and Galaxy had stronger unicorn/pegasus magic than the average pony, since their mother was an alicorn. Twilight and Flash, with the help and support of friends and family, were able to manage these bursts of magic. Raising their children went fairly smoothly after that, up until one fateful day.  
When Galaxy was a teenager, she and other royal guard cadets were put through “Hell Week”-an entire week of intense mental and physical exertion with less than four hours of sleep. The cadets were exhausted as Shining Armor screamed orders at them with Flash by his side. One of the drills involved shooting cannons at targets. One of the cadets, Turquoise Edge, was so sleep deprived that she accidentally misaimed her cannon. It shot straight at Flash, and just like that…he was dead.  
Galaxy was horrified as her father perished before her eyes, and exploded in anger at the pony responsible. Twilight and Dusk were heartbroken as well when they got the news, so the three of them took leave from their responsibilities to mourn. Turq was pretty guilty and tried to apologize, but Galaxy would hear nothing of it. She was let back into the royal guard because Shining Armor knew it was just an accident, but things were still tense between the two young mares.  
After that day, Twilight and her kids found a new normal and adjusted to life without Flash. Twilight, while she missed her husband, eventually opened her heart again and fell in love with Cheerilee, a local divorced mare with two kids of her own. Twilight’s remarriage gained mostly positive feedback from their kids, albeit some complaints from Galaxy, but they were happy together. It took awhile for both Twilight and Cheerilee to settle into the perfect, lasting family, but it was worth it and they are ready for a future together.