>Twilight and Shining have been fooling around for years
>Nothing too serious, maybe only having sex a couple of times before Shining began his training as a guard
>Twilight discovers that her parents, Night Light and Twilight Velvet, are brother and sister
>(I don't know how; maybe she looks through some old archives and finds her family tree; maybe she had a really awkward family reunion with an uncle who tells family secrets when he drinks too much)
>Twilight confronts them
>Her parents look at each other, shrug, and just casually remark that it's how they've kept magic strong in their family for generations
>Velvet takes a sip of her tea and asks Twilight when she can expect to see some grandfoals from her and Shining

>Twilight and Shining are not too hot on the idea of making foals together
>In the words of Twilight, "That's gross."
>Twilight Velvet decides to take the matter into her own hooves.
>She isn't about to let generations of tradition be broken because two ponies are being stubborn, dammit!
>Cue Shining waking up in the morning to a hoofjob and a cheerful "Good morning, dear!" from his mother
>Cue Twilight having to be really careful of where she sits lest her mother slip a vibrator underneath her at the last second

-----------------------------------------------------------

>You are Twilight, the tired and grumpy, and you are walking into your kitchen for a few big mugs of coffee.
>You'd been up all night waiting for the tell-tale sound of creaking floorboards that would herald her mother's journey to Shining's' room.
>And then....
>....the bad-touching would commence.
>She would try and get Shining all horny with some hoof (or mouth) action, and then shove him out his bedroom door and towards YOUR bedroom door.
>What would USUALLY happen is that you and Shining would either just chat for a few hours or read a bit before bed.
>It was tempting to have a quick fuck, but knowing your mother, she probably has a contingency plan for that to ensure that the...
>Erugh.
>..."family tradition" carries on.
>Who knew that there were spells to manipulate the appearance recessive genes and the various unfortunate effects of inbreeding?
>You do, that's who.
>That's how you family has been making brothers and sister make foals together without eventually pumping out deformed retards.
>15 generations.
>Just... Ew.
>You sit down onto the kitchen table and your heart skips a beat when you realize that you didn't check for magical vibrating dildos.
>....There's nothing.
>No vibrations; no phallus filling you up nice and snug; no magical aphrodisiac coating the tip of said phallus...
>....what?
>From beside you, your mother happily moans into her cup of coffee.
>"Mmmm.... good morning, love..."
"Good... morning, mom..."
>What's she up to?
>Velvet smiles a sleepy smile and goes back to reading her paper.
>"What are you up to today, sweetie?"
>I'll play your game, you rogue.
"Not much, mom. I thought I'd study for a bit and then spend some time with Shiny."
>Velvet doesn't even blink
>"That's nice, dear."
>The silence that follows is unbearable.
>After barely 15 seconds, you explode.
"Well?!"
>Velvet doesn't even look shocked at your outburst.
>"Well, what?"
>You narrow your eyes at her, but Velvet doesn't react.

"When are you going to try and convince me to let Shining knock me up?"
>Velvet looks confused for just a moment (that bitch, she knows what she's doing) and then shrugs.
>"Oh, that."
>OH.
>THAT.
>"I've given up, dear."
>Your mind goes blank from the shock.
"What."
>Velvet chuckles at your dumbfounded expression and pats your cheek fondly.
>"You and your brother have made it very clear to me that you don't want to have foals together."
>She goes back to reading her paper.
>"I could tell that you were starting to resent me, so I've decided to stop. No family tradition is so important to me that I would be willing to risk my relationship with my little foals."
>R-really?
>It's finally over?
"After all this time?"
>Your mom gives a cheery (if absent-minded) "Mm-hmm!" as she turns the page of her paper and brings the mug of coffee to her lips.
"After all those mornings where I've woken up to you shoving your tongue down my pussy? After all those mornings where you've made Shining wake up to the sight of himself cumming on your face? Weeks of being careful where I sit? Weeks of not even being able to let off some steam with Shiny's fat cock because I'm certain that you'll somehow sneak in a fertility spell before he cums?!"
>You work up a head of steam, much to your mother's amusement.

"Do you have any idea how frustrating it is to just have oral sex at my disposal? I might love licking Shiny's balls, but I need what's inside of them IN me!"
>You glare at your mother.
"And now you're just.... giving up?!"
>Velvet puts down her paper and rears up on her hind legs, resting her forehooves on your shoulders.
>You didn't even realize that you'd stood up from your chair at some point during your rant.
>Velvet gently pushes you back down into a sitting position.
>"Yes, love. I've decided that since you're too stubborn to budge on the matter-"
>She somehow makes it sound endearing instead of frustrating.
>"-I'll just take your place and continue on the Sparkle family line myself."
"Goo-"
>What.
>WHAT.
>"Soon I'll be nice and full of your brother's foals," says Velvet, fondly rubbing her tummy with this big dumb smile on her face, "I can't wait to finally fuc-"
>Excuse you?
>Bucking EXCUSE YOU!?
"Woah, woah, woah," you exclaim, pointing an accusing hoof at your mother and interrupting her speech, "Back it up there."
>Velvet looks confused.
>"Whatever do you mean, dear? If you won't do it, then it's up to me to have Shining Armour cum inside of me and make me nice and pregnant."
>And take YOUR BBBFF away from you?
>You feel an unexpected protectiveness run through you.
>If this hussy thinks she can steal your very special brother away from you, then she's got another thing coming.
"He's MINE!"
>You stand up and stalk out of the kitchen.
"If anypony is going to have my brother's foals, it's ME!"

>Be Twilight Velvet
>Be listening to your grown-up foals moaning through the walls as they make love
>"C-C'mon, Shiny! Knock your little sister up! Gimme your foals!"
>Be smug

>8 months later
>You try not to look too pleased as your daughter rubs her swollen belly and smiles smugly at you