>>723035438 (OP)
It really depends on how they characterize them in their relationship. I think the best route is Sage sees Metal Sonic as a brother but Metal Sonic is just filled with envy, jealousy, and just has a general hatred for her similar to Eggman.
She is quite literally everything he aspired to be and more. Becoming the new Heir of the Eggman Empire without needing to resort to betrayal, free to use the Egg-Net however she desires, she has free will and free reign but chooses to continue to stay with the incompetent Doctor.
I feel like he would just think "Why?" when it comes to everything about Sage, to the point of anger.
Sadly I think we'll only see that relationship explored in the IDW Comics at least until Movie 4.