So you want your cake and eat it too
She may just want you as backup, afterall they are still human and need care and concern. But how many backups she has is another thing
It could probably backfire on you, this trying to swing both sides at the same time. Since your butt itchy and go and say the gf thing already, why don't you just keep her at arm's length and gradually distance yourself from her. Save yourself future troubles.