Long story from the source, also the reason behind the implied incest tag.
 
It had been quite a few months since the wedding of her son, Shinning Armor and her now Daughter In-law Princess Cadence, Velvet was lounging on her side in bed. Her swollen belly reminding her that she was still steadily on track to making sure her only daughter; so far at least; had a lbbff or lsbff. She still remembered Twilight yammering on about how she was losing her brother and this or that… which brought her back to the day she’d taken her motherly instincts back into her hooves.
 

 
 
The wedding preparations were still being made and Chrysalis still hadn’t gotten the real Cadence out of the picture. In a low in the planning Velvet had asked Shinning to come to her room since he was visiting his parents for a bit. His father had left with Cadence a while ago, something about dress colors since Cadence still didnt have a bridesmaid.  
Velvet approached him and explained everything and what she wanted to do. However, the reaction was just as she had expected, not only was he timid about it but… quite reluctant to do so.  
“Mom… I-I’m your son, a-and Twi’s not losing me at all! I mean if anything she’s gaining a sister!” He pleaded, but slowly his awkward smile began to waver and fade as she… of all things, cradled his head in her arms and pulled him to her bosom. The heat as well as his own cheeks bleaching bright red. “…M-mom…”  
“Do this not for me honey… but for your sister… I can’t bear the thought of her… well, without somepony that she cares for in such a special way.” Unbuttoning her blouse, and with a fumble of words from Shinning himself… he gave in, feeling that loyalty as well as… some deep inner want to always do… this… with his own mother.
 

 
 
Back to the present, Velvet sat up now with a huff and reached out for something, a book. Her slender fingers leafing through the pages and trying to find a specific paragraph she’d read. All things considered her own husband… hadn’t really been cutting it for her cravings or… needs. He was still young to her yes but… what she wanted was ‘her’ little boy. Gnawing at her lip as she fret to find the page before heaving a sigh of relief as she came across the very page she’d been searching for. “The Emperor Prince of the Crystal Empire, as well as the Empress Princess, are not limited to being Sire or Dame to one other… in order to keep the family line ‘strong’.” A smile dancing over her lips as the glass from her reading specs glimmered a bit.
 
A knock on her door did signify that her son had arrived and she knew exactly why. “Mom~~” Came her sons voice in a slightly concerned tone, “-have you seen my book about the Crystal Empire? Cad-Ca..” The stallion had knocked yes, but he also didn’t wait for her to respond before barging in on her… while she was in the nude and laying up top her bulging middle. His face flushed profusely as he fumbled for some words that he just couldn’t find. She held up her hand to quiet him despite his fumbling.  
“Oh? You mean this don’t you~?” She said as she held up the book before tucking it under her breasts. The blue maned, white coated stallion fumbled in a fluster with his words before she stopped him again. “As the Emperor Prince… you have specific perks, like not being ‘just’ Cadence’s Sire…”
 
His ears perking up and clearing his throat as he spoke in a low tone, “H-how many know about…” Looking down toward her full belly. “Just us, Twi thinks it’s just me and your father.” Sitting up as he clambered onto the bed with her. His hands being guided to her bulge and speaking in a sultry tone, “I’ve been wanting a specific something for a -long~~ while… and what better going away present to give your dear mother… than what…” She began to lean in slowly, nipping at his neck and up to his ear with each word, “She… really… wants~?”