All of their interactions with each other is just too big of a coincidence. The only male Apple being obviously attracted to a Pie sister, who also so happens to be a farmer and have the same mannerisms. It'd open up a lot of possible side stories. If Marble moves away to Ponyville, Limerock won't have any sisters living at home anymore. And I think it would be a resolution to all the questions about the Pies and Apples being family. It was basically left a huge mystery as to weather or not them being related was official, leaving Pinkie Pie very worried. She's very vocal of wanting to be family with the Apples. If MarbleMac is canon, and they marry, then it would officially join the two families, and Pinkie can finally be a sister (in-law) to Applejack. I don't think they did this just to tease the shippers, I think this really hints to more of their interactions in the future.
Probably not. It's clear Mac needs to stay single simply because he's one of the only single males in the show for them to toy with. To put him into a relationship would be to dampen their options. Not to say I would be at all angry if they do, I'm warming up to MarbleMac.
Country folk already get stereotyped enough for dating their cousins!! I hope they aren't related. As to my thoughts, it probably won't become canon though.
Oooh no, there's a possibility for two characters interested in each other to be very distant relatives. How terrible. Disastrous, even. Who knows, they could even turn out to be *gasp* the same species! The horror! The horror!
Definitely not. Yeah sure they had some little moments of how they were reacting in somewhat a romantic way. But still, I don't think it'll be become canon anytime soon.
They're distantly related. If they are even related, and I'm more than willing to believe they are, it very very distant. No one remembers if they are or aren't because it's a distant relationship. I'm really tired of people using they may be related as excuse.