Well, as to the sex, she was still arguing in her mind about how far she'd go when he jerked upwards the first time.
She did start out purely with the intent of obtaining pictures to torment her sister with.
Posed, to make it look as if he were doing things with her willingly.
I've no idea what their history is, that she dislikes her so.
Hopefully that will be explained.
Just as why she got involved with the first chap was explained.
Of course, we just got the "bigger is better" trope in regard to dicks thrown at us.
As a prior post mentioned, some are less sensitive than others, so snugness can have an impact, as well as what all comes into contact.
There's also that vaginas vary in size as well; she may also be big in that manner, in which case it just increases the possibility of inadequate stimulation previously.
Whatever.
What's important is that it just backfired on her.
I made a post a couple of chapters back, a few minutes ago, positing that the author might take the path of her actually enjoying it with Oppa, as that would add even more interesting dynamics to the mix.
Because now, having actually had a fun experience, she may turn into long term competition with her sister, rather than just setting out to torment her.
Especially since it was fun when she was using him as a masturbation device, what might it be like if he were actively involved?
That does presume that he isn't a failure at this type of thing.
And... just what are the oldest sister's true feelings toward Oppa?
How will she react when she discovers what's going on?
Because we all know she will.
(And... will anything develop with his coworker? Again, it all depends upon where the author plans on taking this. But, getting such a good view previously, I suspect the author of having plans for further ecchi things with her.)