So this is my terrible Young Sherlock thought of the day.
We all know that Silas Holmes married Cordelia for her fortune right? Or at least it was a huge incentive. She had the money, she had Appleton Manor which would have come under his control once they were married (hence the importance of the Married Women's Property Act in the show). She had all these advantages ready for Silas to take.
So imagine it. Cordelia meets Silas Holmes in her early 20s (Mycroft is 26 in the show, and I would put Cordelia in her early 50s). Silas isn't rich, or at least isn't anywhere near as rich as her. But he meets this person who is clever, imaginative, witty, loving and adventurous (this person who is probably one of the only people who can keep up with Silas's genius), but also a person who is eccentric, perhaps a little mad, and so is likely misunderstood by those around her, which makes her lonely.
So Silas charms her. It's not difficult, because he likes her, but also he sees an opportunity here. This is someone he can manipulate, someone who is so open with her heart and so lonely that she will blind herself to Silas's flaws and to any nagging doubts she might have, someone whose talents and advantages he can use, and if she ever does try to cause trouble - well, he can just say she's crazy and dismiss her suspicions.
So he marries her, and they spend almost a whole life together before it all goes awry.
So how many years do you think it will take before Cordelia looks up one day and focuses her gaze on Sherlock and James -
Sherlock, who may have the intellect and the scientific curiosity of his father but has the open heart, artistic flair, adventurous spirit and madness of his mother (Sherlock, who at least in this stage of his life is "his mother's son"), and James, who is charming, ambitious, a genius, a man who is always on the look out for an opportunity, who has latched onto Sherlock like he will never let go -
- And Cordelia thinks: Oh no. It's happening again.