If you want to follow this explanation of pony genetics, it's extremely unlikely that Otis and Lily (BOTH being Earth ponies) would have had two Pegasi and two Unicorns (any other combination of partners, such as E/U, and that combination of children would be just as likely as any other combination). Additionally, they would most likely display some traits inherent in Earth ponies (think Fluttershy's fear of heights and talent with animals) as well as some... negatives as a result of growth energy being placed elsewhere (think Fluttershy's fears, her social anxiety, her occasional rage issues, etc).But that's assuming that you want to use these genetics and you're not okay with such a low probability. It's not like this is official or canon and even if it was, there's no requirement to stick with it (there are a number of fan-made creations that contradict canon, either subtly or overtly).All this is is a hypothesis about how genetics in the MLP:FiM universe work based on how genetics in our universe work and the information given in the show. You're free to create your own hypothesis or even dismiss the issue entirely if you so desire.By the way, the person that does Slice of Pony Life had his (her?) own explanation of how pony genetics work, using an explanation involving polyploids (the explanation isn't... complete, but it might be something into which you'd like to look) and taking Mr. Cake's explanation as canon: [link] I did some digging into polyploids, and it's a... REASONABLE explanation, but I do take some issue with that explanation. That said, if you want to use an already existing explanation for two Earth ponies producing two Unicorns and two Pegasi, then that's an option there.