Im new to this game, and one thing I am having a problem with is sector ownership. How can a sector be owned by the Yor, when I own the only 2 star systems in that sector, and all the starbases in the sector are also mine?
They have more influence than you there. It means they probably have stars/starbases in neighbouring sectors, whose influence is big.
They may also have influence resources mined.
You should check the influence value (small button in the top-right of the main map).
It is very possible to flip planets by building bases in adjacent sectors.