First off, it's Tate, not Sergey. Sergey has no HiME incarnation.
And the whole thing was supposed to be that Tate was Mai's MIP from early on, long before Takumi died. Mai fell for him but denied it because she felt like she was supposed to dedicate herself to being Takumi's surrogate mother because of her guilt over feeling responsible for his condition/their mother's death and because she didn't want to hurt Shiho.
Not that it really matters either way because they establish in the end that the MIP being alive isn't a prereq for the child to be summoned regardless.