ISFJ Mum (SiFe) + ISTP Dad (TiSe) = INFP Me (FiNe) + ESTJ sister (TeSi)
ISTJ Grandfather (SiTe) + ISTJ Grandmother (SiTe) = ISFJ Mum (SiFe) + ESTJ (TeSi) + ISTJ (SiTe) aunts
ESTP Grandfather (SeTi) + ISTJ Grandmother (SiTe) = ISTP Dad (TiSe) + ESFJ (FeSi) + ESTP (SeTi) + ISTJ (SiTe) uncles
Basically there's a whole lot of Si but no dom/aux Fi or Ne in my family. I must be a mutant.
But seriously, I believe functions are just like other genetic traits; they have dominant and recessive genes. I guess I got the recessive genes.