Even if both Tobirama and Minato had space-time ninjutsu, there's no way they were related. Tobirama was of the Senju clan, and Minato of the Namikaze clan (if it even exists anymore). Anyone who thinks Minato is a Senju is off by miles; it's just like saying Hiruzen was a Senju - that's wrong. Also, Tobirama died before Minato was even born, so that's another way we can cross off relation between the two (student and teacher). They have nothing in common that ties them together, except for the fact both were Hokage and their space-time ninjutsu, which might be different still. But as far as them being related, I do not think so, in any sense of the word.