((Dude, they’re fourth cousins twice removed by a fifth cousin. At least… if they are related. It’s still unclear.
Let’s check the definition of incest:
Sexual relations between people classed as being too closely related to marry each other.
- the crime of having sexual intercourse with a parent, child, sibling, or grandchild.
Key word: closely, as in a sibling, parent/child, and perhaps a first cousin
Here’s a chart from Reddit:
Not sure how accurate this is relationship wise (I’m sure there’s a lot more ponies in between, and there may be a relation through marriage involved), but in any case, they’re too far removed for anypony to even care if Marble and Big Mac date (heck, their genes are radically different enough for any hypothetical offspring to come out completely okay).
It’s not the same as Applejack and Braeburn being shipped, because being a first cousin, Braeburn is more closely related to AJ than Pinkie (maybe) is (and yes, I am uncomfortable with the AJxBraeburn ship).
Who the heck in real life even knows who their fourth or fifth cousins are? I barely even know who my Mom’s cousins are! And even if a married couple find out that they’re distantly related, they wouldn’t even care, because it’s distant.
If Igneous or Cloudy had been one of Granny Smith’s foals, I’d be against the MarbleMac ship. But, seeing as their relation is too distant for them to have even known until Pinkie looked at that family tree, it’s fine.
TLDR: Simply put, if a relative is a parent, sibling, or first cousin, then it’s incest. Big Mac and Marble are too distant to count.
If Pinkie and Applejack hadn’t been throwing around “cousin” so liberally, we wouldn’t even be having this discussion.))