Once again, we were played like fools. The evidence was right there.
The reason the security guard looks like Rukia is because she's a copycat. She wanted to get noticed by Rukia, but once she discovered somehow that Rukia wasn't the murderer, what better way to win her affection that beat the real murderer to the punch!
The brother has an M.O. of throwing people off balconies, something less likely for a woman to do, thus she poisons her victims.
Poor brother, he was mocked, pitied, but then saw an opportunity to do good, and was quickly led astray, barreled down the wrong path and used his sister to fuel his own twisted ego trip. Mind you, just because he had a tough upbringing it doesn't excuse his actions, nor Rukia's actions in instigating his hero complex for money.
That or she just has craptastically abysmal taste in men while simultaneously being a hoe from whom even children are not safe.