@Flying Pancake  
See my family tree outline below.

Weren’t they something like ninth cousins? And isn’t that functionally not related?

Okay, let’s see my headcanon family tree and the shortest link I can find between Apples and Pies:  
-Mud Briar’s father is Puppysmiles’ great uncle  
-Puppy’s grandfather (brother of said great uncle) is married to Cardinal Point  
-Cardinal Point is Featherweight’s older sister  
-Featherweight had a foal with Scootaloo at some point  
-Scootaloo dated Babs Seed at some point  
-Babs Seed is Big Macintosh’s cousin  
That’s what I drew on my WIP family tree. And yes, I managed to squeeze Puppysmiles in there.  
-Alternatively, Puppy’s frozen eggs were used by the alicorns to produce viable foals with help from Slug. Slug is a distant descendant of Babs Seed.

@Background Pony #78FC  
Here’s Jim Miller’s brief answer on Marble Pie’s future love life: https://twitter.com/TheBiggestJim/status/1183446004776890368

Don’t worry. Jim Miller also tweeted that Marble Pie will find her special somepony someday. Now it’s up to the artists and writers behind the Season 10 comic books to show Marble Pie meeting her special somepony and finding her happy ending.

i fucking called it.

yuup ouch

@Background Pony #4871  
On the brighter side, Limestone and Holder’s Boulder have been happily married for years and have three beautiful pebbles together.

Good job Pinkie. Let that sink in after you realize that you married Cheese Sandwich while your younger sister is 40 year old and still single working on a rock farm

…but they wouldn’t have even met if Pinkie didn’t think the they were related in the first place.