My estimates assume that they were their given ages in 1996, when the manga ended, rather than when it began. Takahashi has always been fond of time compression and/or 'neveraging teenagers'. If I count from when it began, they'd be:
Akane - 37
Nabiki - 38
Kasumi - 40
I know I'd like to pretend that Inuyasha never STARTED, but that's a different matter entirely.