Ever since I first read the manga, I wondered if Sakaki's sister and Kenichi's mother were the same person. Now there is no confirmation for this. If anything, there's confirmation against it considering the sister is supposed to be an office woman while Kenichi's mother seems to be a homemaker. That said, I do want to (in no particular order) give my reasons for thinking it in the first place.
1. We never see the sister's face, leaving it open that they could be someone we've seen before.
2. The sister has a tall, slim, but well endowed figure similar to the mom's.
3. The sister is only ever seen in flashback, implying that Sakaki doesn't keep in touch with her and thus might not know her familial status.
4. I felt that his eyebrows and Kenichi's eyebrows were similar, and thought that may be because they're related.
5. The sister isn't bothered by seeing guns, despite living in a country with strict gun laws. The mom is married to a guy who's a skilled gun wielder and is also not fazed by guns. Maybe, as the sister, she was already dating Kenichi's dad and was used to his past time?
6. Sakaki's has stated that older sisters are scary when he sees Castor and Pollux together. He has an older sister and was presumably scared or intimidated or obedient to her in the past which supports that statement. Kenichi's mom has both knocked out and intimidated her husband without hesitation, implying a pattern of behavior, possibly something she practiced on a little brother.
7. We never know the sister's first name nor do we know the mother's maiden name (at least I don't), again leaving the possibility open that they are one and the same.
8. No one who has met the sister has also met the mom, including Sakaki, so there was never a visual confirmation that they were not one and the same.
9. Kenichi's mother has no issue resulting to violence, and has very little issue with her son doing so much to learn martial arts. Is it because she's been down that road before with a little brother?
10. In the future, a grown Kenichi's arm looks similar to Sakaki's arm, despite the fact that Kenichi has always had a slight build closer to Akisami's or Kensei's, not to mention that his body was being redesigned from the inside out by the smaller two masters. So what caused this seemingly unexpected growth? Maybe a growth spurt that made him look similar to an unnamed uncle?
Now there are a few other things that don't work with this theory, like the age difference between mom and Sakaki, the idea that Sakaki wouldn't know he had a nephew and niece and brother in law, the different hairstyle of the sister compared to the mom, and I already mentioned the different jobs. But I do want to know what you think of this idea. Also, I want to know if I'm the only one who ever wondered about this.