Yeah there is pretty much no support for this idea but screw it I like the idea and people think with their emotions nowadays instead of logically so let's roll!

My idea is this, Ghira used to be the leader of the White Fang and Sienna was his replacement leader, so I think that could mean that the two of them were close or had some sort of relationship in the past. With this in mind I don't think it would be crazy if Rooster Teeth decides to give Blake a half-sibling who is the child of Ghira and Sienna.

Again, I know there really is no real evidence for this, but I like the idea and just wanted to put it somewhere.