>>42733442
I know, like I said, it's very unlikely. And in this case, even more so due to some of Igneous' characteristics being present in both Marble and Limestone, as well as each pie sister (pre-cutiemark) sharing design elements.
Infidelity just isn't a logical explanation for the way Pinkie was born when you have other, more reasonable, explanations like: recessive genes (probably from the elusive ''Granny Pie"), coat and/or mane color not being consistent across generations for other ponies, magical pony bullshit, etc.