Originally Posted by mr. peasant
Sounds to me like the obvious answer to that is to come up with a good purpose for her to move to once the subplot she originally served is sufficiently explored. In other words: character development.
No sense introducing introducing a whole new character if an old one could do just as well.
As for Oliver being 'the last Queen', there's no indication that this is the direction that the show intends to take. More to the point, getting rid of Thea, Moira and Walter would be getting rid of half the show's current supporting cast; which would be a major retool of the show - completely changing the character dynamics and story balance.
Seeing as how the show is pretty successful at the moment, I don't foresee the show making such a drastic change in a short period of time (i.e. not in the near future).
There's only so long they can drag the family stuff out before it ultimately hinders the protagonist from developing.
Their purpose is relevant to his origins and to support that story. Basically, the 'Arrow Begins' phrase.
It's not like Arrow will be living with his mommy and sis during his glory days. It works now but they have to cut the ties eventually.. especially with Moira. She's a villain. Keeping Oliver oblivious to that for too long ruins it and makes him look inept.
Also, the show seems to be hinting that the Queen empire is destined to fall given that it was built on the suffering of others and corruption... and it's under the control of a corrupt head (Moira). I think they'll follow the storyline from the comics where Oliver loses all of his fortunes and becomes a street avenger... fighting for the common man.