I'm not really sure about Karin having a crush on Motoaki... We can gather from chapter 1 she doesn't really have a father figure she can say she loves, but everything else reads to me as her feeling embarassed about an older man acting like a caring father would, which is closer than she's used to, but without any romantic tone about it.
Then again, I'm sure people have confused such feelings with romantic ones before, so there's that angle too.