[Jawful]

oneesan is perfectly acceptable to call a young woman, at least around here. I regularly get called oniisan. You can also call old women obaasan and men ojiisan. ojisan and obasan are ok for obviously middleaged men and women, but better to use onee and onii for people who are in their thirties. They are still pretty age-consious and like to pretend to still be in their 20s.

But I'm with MMM in that oneechan is pretty much the beginning of a pickup or at least with a "you're cute" tone, even if no nampa is taking place. Usually the person saying it isn't sober. I've never said it sober, anyway, heh.