Plot holes (spoilers)

Good questions -- this movie sort of fell apart for me at the end. Another thing that seemed odd was they set up the situation with Kitty trapped in the crypt and the only one who can save her is Herbert, who is crippled and must limp all the way there. This seemed like the perfect set-up for a tense sequence with Herbert limping trying to get there in time while Kitty is getting ever closer to being drowned, but instead her boyfriend calls the cops and abruptly we cut to the cops instantly being at the house opening the crypt (and Herbert stupidly standing in the way and dying for no good reason). Seemed anticlimactic.

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- Kitty's older sister, Franziska, resented that she wasn't the main heir and wanted the grandfather's inheritance for herself & her husband (Herbert). So she enlisted some of Kitty's colleagues at the fashion design company in her scheme, which included Lulu. Franziska perhaps gave Lulu the necklace (or a facsimile) as a gift. The writer likely added this to the plot simply to cast suspicion on Lulu and away from the real killer. It's a red herring.

- Evelyn never swam back to shore in the flashback. She was unconscious while floating in the water with her face up. When Franziska recovered the body, she noted that Evelyn was still alive and so banged her head against the rock to kill her, unbeknownst to Kitty.

- Rosemary is ultimately revealed to be Kitty's true sister while Evelyn was adopted by the grandfather, Tobias, from a "peasant" family. He presumably did this to negate the family curse of the Red Queen. Rosemary turns out to be the cackling slayer in the red cloak, wearing a mask to appear like Evelyn from a distance.