Here's my unobscure Holmes theory: he has a sister. Maybe multiple I don't know, but I do think he has at least one. (Line in the Solitary Cyclist-i think- that leads me to think this, cannot be bothered to find it right now so I'll add it in later)
My opinion on this theory however, is that most people (honestly I think all the big offenders that I can think of are men so it's less "people" and more "men") who give Holmes a sister are entirely incapable about being normal about her (see Eurus, Beatrice and the sister in the Young Sherlock book series). She has to be hyper-intelligent and evil/insane and she has to be some sort of great big secret and the Holmes life revolves around her, because she's not really a character she's a plot device to explore Holmes and cause some chaos and give the series ways to do that. But like, what if she was just a woman- yeah she's a tad odd, but she's a Holmes she's going to be, and she's smart, but she's just a woman- who doesn't get mentioned all that often cause Holmes just doesn't talk about his family. What if she's off living her life, turning her intelect into something she likes and enjoys (and isn't detective work, because she's not Holmes, she isn't defined by her brother, she exists in her own right) (the fact that her brother is that famous detective is something she hardly thinks about because, fundamental it's much funnier to think about that time Holmes blew himself up during a chemistry experiment and she can tease him for that). Anyway. What if people thought about a Holmes sister as existing separate from her brother's?