How do resembling characters affect a mystery?

I’m so confused about the character of Lord Saint-George. He is Wimsey’s nephew whom Wimsey must always bail out of trouble and give money to despite the fact that the Wimseys are an aristocratic family. His parents provide him with a meager amount of money while he attends Oxford and thus prompting him to constantly beg Wimsey for money, especially to pay his many debts. Wimsey complies, showing Miss Vane a different side, especially when he pays for Lord Saint-George’s medical bills, which was the result of recklessness. Lord Saint-George constantly acts out to get attention, including making inappropriate gestures with Miss Vane, whom he has recently took to calling an adopted aunt.

So I know that his character helps convey another side to Wimsey and allows Miss Vane into his personal life—one which she took no interest in before—showing a different side of herself as well. At the same time, it’s odd because Miss Vane keeps mentioning how alike they seem. She first met Lord Saint-George, when he bumped into her; she mistook his voice and hands for Wimsey. Now Miss Vane comments that “These family resemblances were unnerving.” It’s odd that she would describe their resemblance as unnerving, suggesting something sinister and dark about the two. (Don’t family members resemble one another?) This leads me to believe that perhaps they are part of the problem and disruption at Shrewsbury College. However, I don’t see how that could be possible considering the fact that Wimsey is in love with Miss Vane and Lord Saint-George doesn’t seem to have any interest in doing such things that don’t involve money or making a scene (of which people know he made the ruckus). At the same time, he has offered valuable information claiming that one day, a woman grab him from behind at night, hoarsely whispering into his ear, which Miss Vane assumes to be the Pen Poisoner. However, Lord Saint-George could be lying since nobody witnessed this encounter and because he told nobody else of this incident in fear of them mocking him. Still, I don’t see how he could be the culprit or be involved.

When mysteries introduce characters that resemble each other or who could have faulty testimony, it always throws people off (keeping the mystery aspect alive). People who look like each other can frame one another as can people with false testimony. Lord Saint-George’s character seems so suspicious to me now because of his similarities to Wimsey (and strange relationship with Miss Vane). Sayers has thoroughly confused me. (She’s must be doing her job!)