Seina has never met Mihoshi. I still find this very suspicious and I definitely don't think it's just because Mihoshi never goes into the shop when we know that Mihoshi does a lot of the shopping for the house. Especially since he's met Noike as of gxp ep 17 when Mihoshi had been in the house for nearly 2 years by this point and Noike had only been there about 8 months. I definitely think Mihoshi's good luck made it so she stayed far away from Seina and his bad luck, like two magnets that can't touch each other. Notice that when Mihoshi arrives back to the house at the end of the episode, Seina is grabbed and has to go on a mission with his girls.
'm almost positive that Seina's bad luck is the result of the Chousin's purpose just like Mihoshi's good luck. In the end of GXP, Airi says that Washu's conclusions about Seina come from her research on Mihoshi. Also, in GXP17, Seina says he's never met Mihoshi. This could simply be because Mihoshi was on patrol or on a mission a lot OR whenever she did the shopping (which she usually does), Seina wasn't in his family's shop. However, could it be theorized that Mihoshi's good luck made it so that she stayed far away from Seina and his bad luck whenever possible? Like two magnets?
They actually have met, and in true Mihoshi fashion, she slept walked into Tenchi's room naked and got in the bed with Seina since he was sleeping in Tenchi's bed.
(http://i.imgur.com/RkrKyA0.jpg)
Also it is possible Seina's very existence is the counterbalance to unusually high luck Washu's descendants have. Much in the way Misaki is the manifestation of all of the negative energy created from the choushin manipulating the universe.