I would like to ask what would be the relationship between Seija and Suika/Yuugi?
There's a lot of ways it could go. Amanojaku are
kinda like oni (or at the very least, Seija could pass for one as long as she wasn't asked to prove it with muscle or liver strength) so on one hand they could be friendly. On the other hand, oni are extremely honest and never lie. Seija and maybe amanojaku in general, by contrast, are contrarian by nature and as such will lie their heads off. In this scenario, neither Suika nor Yuugi would be very fond of Seija. There's also the fact that she's
probably friends with Shinmyoumaru, who was descended from the oni-slayer Issun-Boushi. If they dislike Shinmyoumaru enough, they may also just automatically dislike Seija by extension.
On the other hand, depending on the person portraying Seija and how their Seija feels about Shinmyoumaru, the oni may end up tolerating or even liking her as a valuable way to get directly to the little princess. The Seija in question would have to be quite a backstabber for this to happen though, and Suika/Yuugi would have to be pretty patient and tolerate her contrarian crap. By contrast, if the oni could care less at this point, it can be all up in the air since Suika, Yuugi, and Seija can all be portrayed in various different ways.
There is one scenario - although probably unlikely - where none of the above quite applies. Let's assume Yuugi is the type to straighten people out with tough love. If she decided to do this with Seija, it could instead be that Yuugi
likes Seija.
She's already got quite a history in fanon of pairing up with tsundere types, so why not? Depending on the Seija and how well you can work a double negative, the like/love may eventually become mutual. This is akin to classic Kaguya/Mokou stories in that Party A (Mokou/Seija) initially dislikes Party B (Kaguya/Yuugi) but eventually comes to like them, typically with Party B already having unrequited feelings (friendship, love, etc.) for Party A. This could also be a scenario where Yuugi's a brash-but-good-natured onee-san type toward Seija, who would no doubt have a tough time in Former Hell with all those oni walking about.
However, like I said, it all depends on who's portraying the girls in question and numerous other variables as well, so there's no solid answer to your question. If you're asking about how these girls would get along in canon, I'm guessing Seija would dislike them from the start (her plan having been to overthrow the strong and all) and provoke the two oni girls. From there, Suika could either mutually hate Seija or just not care, and Yuugi would probably keep putting the amanojaku back in her place, mainly for fun. Though there is the fact that Seija can make Yuugi's sake even drop. Then it's a whole new ballgame.