Would be funny if the sister actually knew what they are doing... And yes, I laughed at the earthquake bit like a madman...

These two are made for eachother.

The girl is a keeper cuz she is saving all that money towards her wedding with oni-san!

That's an interesting title.
I like how at first it just seems like the usual paying for sex kind of thing, but in reality, she actually likes him and is waiting for him to confess while she keeps the money she gets for their wedding fund...She's just so adorable.
No worries about your paycheck, my man. It's going towards a good cause, that being your future together! Seriously though these two go together so well, they'd make a cute couple...But the way things are going, her waiting for him to confess while he thinks she's a slut who just wants his money...I think it'll take awhile, but I trust they'll get there one day.
Also...'Earthquake', heh. I wonder if she does actually know what is going on and is feigning ignorance or really doesn't suspect a thing.

I really hope this gets a second part.

I like a girl that plans for the future.

Well That Was Something Completely Different: seems both 'Onii-san' and Sister's Friend "have feelings for each other", yet they're presently carrying on a strictly "sex for money" arrangement, and thinking that's all there is to it.

Could well be a case of Art Imitating Life, where "... the names have been changed to protect the innocent."