As a Wonderbolt, Rainbow Dash began seeing Soarin more and more often. At first, they were just friendly acquaintances. As time went on, however, they grew closer and closer as friends until they developed a liking for each other. This is partly because Spitfire deliberately paired them together in their Wonderbolt routines.
One day, Rainbow decided that she'd had enough of the casual flirting and asked Soarin on a date. From that day on, their romance blossomed as they went on more dates and became an official couple.
After a couple years, Rainbow proposed to Soarin. Of course, he said yes. Rainbow and Soarin got married, moved to Cloudsdale, and lived a few happy years as a couple.
Rainbow eventually decided she wanted somepony to carry on her and Soarin's awesome legacy. She wanted to start a family. About a year later, their firstborn son, Lightning Bolt, was born.
They lived as a family for about three years until Lightning Bolt started asking for a sibling. Rainbow and Soarin agreed that they'd be up for having another child, so Rainbow got pregnant again. After 11 months of mood swings, Firework was born. Lightning Bolt was happy to have a baby brother, and Firework was loved by his whole family, but Rainbow insisted that she was not having another child.
And it was so for about five years…until, after a night of fun, Rainbow got pregnant again. She wasn't happy about this, but she and Soarin decided to go through with the pregnancy. After another hormonal pregnancy, Rainbow and Soarin's daughter, Paint Swirl was born. They were very happy to have her, despite the fact that she was unplanned, but Rainbow made sure that she would not get pregnant again.
Rainbow Dash and her family eventually moved back to Ponyville, and they've been living a pretty awesome life since.

Also, Tank has been a significant part of their family throughout the years. He may be old and slow, but he has been a faithful friend and companion to Rainbow Dash and her family through thick an thin.