My headcanon for Rarity, Spearhead, and Coco Pommel is essentially two good friends living together with the babies while dad visits regularly.
 
 
How they met: At the time that Rainbow Dash was being falsely accused of sabotage in the Wonderbolts, Rarity had gone through a lot of trouble to track down the pony framing her. One of the ponies they questioned during the investigation was Spearhead from the Royal Guard. Rarity had charmed him and his co-guard quite a bit during this encounter which left an impression on him. Later on around the time that he had left the royal guard and opened an art exhibit, Rarity had come to see it along with some of her artistic Canterlot friends, one being. Spearhead recognized her although it took her a moment to remember him (he looked different in armor).
 
 
Relationship (brief): Their relationship was extremely passionate and fast-paced with both or their work suffering due to their enamorment. They were generally very careful intimately, but Rarity still ended up becoming pregnant which added a lot of strain to things in addition to their suffering careers. Rarity had neglected a lot of her work leaving her assistant manager irritated and run-down, whereas Spearhead turned down a big opportunity to move elsewhere and set up a gallery. The two discussed it for several weeks at length, before coming to the conclusion that, long term, this was not what they wanted and the relationship, despite how much they loved each other, was too hard to make work. Nonetheless, while Spearhead was not ready for children, Rarity elected to keep the baby (or babies as it turned out). She and Spearhead stayed in touch for the following years and, after he was established and situated with a permanent gallery, they elected to start allowing him to visit regularly. It took a long time since their feelings for each other were still lingering, but by the time the girls were four, they had both moved on with their lives and could handle the visits better.
 
 
OFFSPRING  
Coco Pommel: She and Rarity are close friends who mutually benefit from living together. In Rarity’s case, she is too busy and career driven to settle down currently whereas Coco, being a-sexual, appreciates having someone to live with in a non-intimate manner. She loves children and enjoys being a part of the household. It brings her immeasurable joy to be called “Tata Coco”.
 
 
Ma Petite “Tati” (pegasus) is the youngest sibling (by six minutes) who is very gentle and shy by nature. She loves drawing and is the cat’s favorite family member. She and Opalescent spend many an afternoon doodling in the bay window. She gets along really well with Auntie Fluttershy and is besties with her son Snowberry since they are similarly gentle souls who like animals. Aspires to be a veterinarian or conservationist. Rarity often feels jealous of how much her daughter seemingly relates to Fluttershy and admires her.
 
 
Her sister Tres Magnifique “Tressie” (unicorn) is seemingly fearless in contrast and particularly enjoys telling scary stories, although her mom’s artistic flare shows up in her elegant prose and verbiage. As she gets older, she picks up writing and goes on to become a celebrated horror novelist. Applebloom’s daughter Ambrosia, who is much younger, devours the books much to her parents’ dismay. When she was little, Tressie looked up to Rainbow Dash because of her awesome scary stories, but once she figured out that Dash was more interested in sports and competitions, she lost interest (much to Dash’s irritation and Rarity’s relief). Nonetheless, it is still likely because of Dash that Tressie became so fascinated with horror stories.
 
 
Rarity gets occasional anxiety about the fact that neither of her daughters is very like her (at least not at first glance), in fact, they’re more like some of her closest friends. Ma Petite is similar to Twilight and Fluttershy, whereas Tres Magnifique is outgoing like Rainbow Dash. Especially early on in their lives, this caused her a lot of distress. She has since learned how to accept their personalities and find smaller details that tie them together.