Technically speaking, if it were true that Dan and Tsunade did have a child, and that child were Minato, then since that would put at least 38 years between the birth of Tsunade and the birth of Naruto (Tsunade and Jiraya having been about 50 years old when the series statrs and Naruto is 12). In that time period it is biologically and chronologically possible to squeeze in Tsunade having a child as well as Naruto being born.
Now while that theory is a pretty funny one, and one i would love to believe, why haven't they mentioned anything about it yet. Actually, why hasn't Kishi ever really mentioned anything about Minato's family? (Apart from Naruto and Kushina of course).
Oh and wouldn't it be weird Minato being hokage, then having his mother be the next one? That just seems a little strange to me, but hey, this series has its moments.
But Tobirama being Minato's father, completely impossible. Great, great uncle, maybe. But father, no.
And so we return to the topic of incest...