1) It's not lewd enough
2) The entire point of the series gets boring about halfway through
As for the first point, even watching the uncensored version there weren't enough lewd moments to keep my interest, and because of the second point I actually began to nod off during the last couple episodes. I really don't know why I kept on with it, except maybe avoiding work or the fact I was generally starving for harems.
Series like DxD or Monmusu maintain their lewdity from start to finish and also have good stories to tell while there aren't cartoon tiddies flopping back and forth. It's fun to stay involved in both senses. Oniichan doesn't have consistent fan service and the "slice of life" feel it tries hard to maintain doesn't present any interesting scenarios. There's a bikini judging contest towards the end of the series that was so utterly boring I don't think it could have been done any worse even if there was a free-entry contest called "design the worst bikini judging contest and win five billion dollarioos." The last couple episodes are also quite heavy on flashbacks, and those aren't interesting either because I couldn't have given less of a fuck how these people met as kids since I just watched ten episodes of them being friends already.
The spicy incest angle is also boring to watch because the brother never once slips up in refusing advances and the sister is obnoxious in all ways. She isn't sexy or alluring or possessing of a funny, witty personality: she's a bratty bitch who never stops whining about everything. Supporting characters in the "harem" (it's weak and nothing at all like DxD, etc) have various angles to them but no depth to speak of. I also think the legitimate loli was gross, although maybe that's just my puritanical upbringing clawing its way through my adult facade.
Why not a 1? Because I still finished it. The premise was decent, and what little lewd scenes there were at least made me pay attention. I don't particularly regret watching either, but I definitely wouldn't recommend to anyone else unless you're like me and just about out of new harem shows that actually have some "plot" to them.
