If the MC is not a futa her surname should be Summerdotter not Summerson
Summer -son (Son of Summer)
Summer -dotter (Daughter of Summer)
back when people were named like that it would be -dottir
This - and as far as I know, Iceland is the only country still doing so - or is that old news and they changed it as well?
So today -son should be perfectly fine.
There should be many more -sons (male and female) out there then -dottirs these days.
At least I never heard of anyone actually having a surname ending on -dottir outside of the context of Iceland.
But then, the name is very much fiction to begin with.
It can't be an old surname because that would mean some Scandinavian/Icelandic guy from the past was named Summer - and back then no modern English word like that seems likely to have been used as a name(if it even existed yet), even less as the name of a boy.
Or, in the very recent past, an Icelandic boy was named Summer (still not that likely, especially for a boy) and later had a son who emigrated to the US (assuming that is where this takes place (didn't play it yet)), cementing the surname, and there he or someone soon later had a girl they named Summer again.
Which of course in theory, unlikely as it still is, could be what happened.
