Aiacco

I see your point, and since we can't exactly go back and ask the Vikings how they did their surnames, we'll never truly know for sure.

However, according to the Hooligan Family Tree from the books, Stoick's father's name was Squidface the Terrible (okay, that's a bit much, even for HTTYD). If they did use the father's name for the second born's surname, then Spitelout would be Squidfaceson.

Of course, there's no real way of knowing whether Squidface is his father's name in the animated stuff. So it could be. Until Cressida Cowell or DreamWorks confirms this, however, we'll just have to speculate

Tentra

Even if Valka was Spitelout's sister, making them brother-in-laws, Snotlout would not be eligible to be chief, like he is frequently alluded to being. Again, because inheritance typically ran through the male side of the family, and his connection would be on the female side (aka Valka), not to Stoick the chief. He and Hiccup would be related by blood, making them cousins, but Snotlout and Stoick would not be related by blood, and blood is what counts to the Vikings, both literally and figuratively.

Guest

Again, like I said with Tentra, even if Spitelout and Valka were siblings, Snotlout could only be heir if he was related to Stoick by blood. Because he would be the nephew-in-law of the chief, not the actual nephew, he could not gain the title if Hiccup died, went missing, was disowned, etc..

LucyBC1990

It is quite possible that one of Stoick's uncles did adopt the name of his wife, though highly improbable. I did base part of this off of the family tree you can look up on Google (which also shows that Stoick and Alvin are actually distant cousins), but because this is mostly for the animated franchise, not the books, I mostly disregarded the fact that Stoick has six siblings (because they never show up in the movies, the TV shows, or the Netflix series.

Guest

While they are not explicitly stated as being relatives, to most fans (including myself), they are. I'm pretty sure we are drawing from the books on this particular point of the fandom, so in a way, it is selectively canon. In many of the fanfictions I read, they are cousins, so I simply go with that. You don't have to; it's your decision, after all. But this was written on the basis that yes, they are related in the animated franchise as well.