I don't wanna generalize, I really don't, but this seems to be consistent among people with incest fetishes.
"Onee-san" can mean both "Older Sister" and "Lady". Oba-san is 35+ (but you'll get your ass beat if the woman is under 45 and you use it. And some over 45 and under retirement age will get pissed too. But at that point it's their problem.) but Onee-san is ANY woman who is either older than you if you're a kid or teenager, or between the ages of around 14 and "oba-san" age.
Good luck with that. Even Japanese people can't agree on exactly what it means. In porn game contexts it can be an older sister or a 20-something in relation to a teenager.
Naturally this makes this game *especially* ambiguous as the main female character is in a school uniform.