What if "Cousin Selek" was a common joke on Vulcan? Like John Smith or Jane Doe, 'Selek' is a common name, so it's not entirely inconceivable that Spock- or any Vulcan- would have a cousin Selek. Or many cousin Seleks.
There are at least three Cousin Seleks at any Vulcan party.
Sarek: I suppose you do bear some family resemblance. Who are you?
Future Spock, panicking: ... Cousin Selek.
Sarek: Ah, yes! Of course! Cousin Selek!
Sarek, internally: My cousin Selek? Spock's cousin Selek? Amanda's cousin-? Amanda doesn't have a cousin Selek... In fact, now that you mention it, I don't have a cousin Selek. Do I? Did T'Parna adopt another child?
Vulcans with their cousins Seleks be like latines with Tias:
"Who is she?" "I heard someone call her Tia Marta" "huh, ok" and now you have a new aunt.
or alternatively: "mum, who is she?" "mi Tia Silvia" "ok" and like are they even related by blood?? is she married to anyone from the family? or was she just someone's neighbour in the 80s and is still being invited for some reason? she was probably friends with grandma right? is this the Silvia that beat grandpa at truco (a cards game)? or Tio Pablo's wife? or another Silvia entirely???
and then when they greet you they talk about how much you've grown and you're like madam this is the first time I've heard of you in my life please don't say that you knew me when I was a kid I don't know how to respond
except that with Cousin Selek there's a 6% chance he's actually you from the future of an alternative reality so he does know you but he's hoping you don't ask him how or when because he can not answer that
Me at family gatherings