Swinwk, I don't think we have to worry about Sienna spreading her gene pool. I'll bet there is a little bit of her in Washoe and Zephyr.
Given the tolerance, allo-preening, and feeding between Sienna and Jina, Sienna is probably Jina's mother and Jina is a 1st year bird with her 1st nest. I believe Karla from the International Owl Center felt that for whatever reason, Sienna did not become territorial. First year birds are pushed out of the territory by their parents by fall. Young GHO's can breed their 1st year and have been known to establish their turf within a few miles from where they were raised. I am guessing that Jina did in fact leave, but did not go far and found a mate in Arie. Sienna may have lost her mate just before nesting season and became a floater. Floaters maintain a larger territory than mated pairs, which may have overlapped with her daughter. For whatever reason, Sienna ended up hanging out with them and did not get a new mate of her own. Jina tolerated her presence, which became both a blessing and irritant for this 1st time Mom. As Shep states, this was certainly an interesting study. We have seen polygamy in bald eagles, but I don't believe it was ever recorded in owls. Owls can lay eggs whether they have a mate or not. Karla's unmated owls do this each year - Ruby, Piper, and Uhu all exhibited nesting behavior and laid unfertile eggs this year at the International Owl Center. Sienna is clearly a very experienced mother. With the owlet on the ground, her extra set of eyes and experience has been very helpful to these newlyweds. Hopefully, at the end of nesting season, Sienna will find a new mate and Jina will continue her life with Arie. GHO's don't always return to the same nest each year, as we have seen at Savannah, Georgia and in OKC, much to our chagrin.
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A fun to watch exchange between Zephyr and Jina starts @ 7:30 PM through 7:56 PM on the DVR. Starts with Jina bringing in a rabbit for Zephyr and ends with Zephyr biting her tail. Not only does Zephyr look like Jina, he is starting to act like her!! 
All going well here. Here's a pic from Tuesday, May 1st of the owlet on the ground...doing very well. I continue to believe this is the younger owlet, Zephyr. Tail feathers are much less developed and barely visible. I think people may have assumed that with this owlet fledging first, it was the older one, but that is not necessarily true. This pic is a screenshot from a video posted by DRI on the Nevada Dept of Wildlife Facebook page.
No worries Swinwk....I know that everyone on chat has been referring to the ledged owlet as Zephyr. I was just posting my observations of feather development. I always thought the smaller owlet ended up on the ground, so I thought it was the younger of the two, which would be Zephyr. I just looked back and it appears that both owlets may have hatched only 1 day apart, so their feather development should be similar. Shep is probably right about the owlet on the ground being male, which would explain why it is smaller and could still be the 1st hatch. To be honest, I have no idea who hatched 1st...lol. 
We may have a female on the ledge and a male on the ground. I always thought males developed faster, but that's in falcons and eagles. Not sure about GHO's. The owlet on the ledge does seem further along in development. The two moms kept these owlets covered up so much after hatch that it is hard to say who hatched first. It really doesn't matter. The owlet on the ledge is being called Zephyr and that's the name it is. 
You know what Shep and Nadia, I am afraid Zephyr is comfortable up there and may delay fledge. I wish you all could have seen the 20-25 minute run of the shenanigans between Zephyr and Jina last night, when she brought in the bunny. Tug of war, stealing it from each other, finally Zephyr had enough of his mother and pretended to swallow it whole!! You should have seen the look on her face when it happened and when she stepped around him to make sure she just saw what she had just seen. Hahahaah!! Then the little devil let it back up and took it to the pantry to eat. She left him alone after that. When he was finished she had a couple bites of it too, on his terms. It ended with Zephyr jumping up on the ledge and biting her tail feathers. She left after that. I think he looks like her and is emulating her quirky behaviors and I wouldn't want it any other way. We now have 2 quirky owls in the world and I love them to pieces. Once in awhile I see Aries with food deliveries directly to Zephyr, but rarely do I get to see Sienna anymore. And only Washoe when they publish a video. As Jina and Aries were the original mates to this nest, I hope they return next season! Would love to see Sienna here too, if she could focus on her eggs, if it were the same circumstances. But, she needs to spread her gene pool to the world too! I wonder if the powers that be will even continue a cam here next season. The gentleman with the initial interest has retired.