Well, regarding the bluff question there are some other issues that can throw some light on it:
-It was never clearly stated in the anime, but the usual interpretation is that all the girls had different mothers, so mitochondrial ADN is not evidence anyway. It can be good to throw some more impressive-sounding thechnical terms in the bluff.
-At the end of the anime, it was made clear that that everyone (Wataru included) knew from day one that Mami was not a sister and were letting her stay there anyway (of course, in this scenario there is Doug who has the resources and inclination to notice that she may be up to something more sinister that just being a down-in-her-luck squatter)
-It was never clearly stated in the anime, but the usual interpretation is that all the girls had different mothers, so mitochondrial ADN is not evidence anyway. It can be good to throw some more impressive-sounding thechnical terms in the bluff.
-At the end of the anime, it was made clear that that everyone (Wataru included) knew from day one that Mami was not a sister and were letting her stay there anyway (of course, in this scenario there is Doug who has the resources and inclination to notice that she may be up to something more sinister that just being a down-in-her-luck squatter)