I doubt the political situation in a reunion would be all handshakes and hugs.
The GTVA is by it's own law the only legitimate authority in Human and Vasudan controlled space.
But back in the times of the GTA, Earth was calling the shots.
Since the GTVA is mostly democratic for all I know, that would leave Sol, or sectors of Sol, with just one voice in the council. Going from total rule to just one voice isn't going to sit well with whoever's governing Sol I guess.
That doesn't mean there has to be a war, but a military and economic - but not political - alliance is the most likely scenario for me.