Raubritter

Three years ago, Karamura Neneko's father died. Since then, she's been living together with her older sister, Nanako. They've been living together happily until recently, when Nanako starts acting strangely. There will be nights that she doesn't sleep at home, and then one day she completely disappears. Not only that, but she quit her job at a company that she had been at for a long time.
Worried about her sister, she remembers a place that her father told her to go to if she was ever in trouble, the Raubritter. Upon arrival, she meets Shinobu and Yokana, cousins that run a detective agency, more-so as a hobby than a career. Since they are both fairly wealthy they never ask for payment. They only help young maidens in need, and since their business is a secret, only people with connections come to them. It turns out that Neneko's father helped them out a lot in the past. The cousins are heart broken to find out that he died, so they are more than willing to help Neneko find her sister.
What has happened to Neneko's sister? Will a special relationship develop between Neneko and the cousins?
(Source: MU)