There are quite a few people that blue water fish in Belize (mostly private trips) but not many charters out of San Pedro. Is that going to be your only destination in Belize?
Last year my fishing group had checked to charter Sea Boots from Hustler last year for the Belikin tournament (this year's tournament is next weekend out of the Belize Yacht Club on AC) but we ended up chartering a boat out of Placencia that is rigged for blue water fishing.
Blue water fishing is expensive (and can sometimes be slow but we have never come back skunked so I guess it is a matter of skill and a good captain).
I think most fishing guides in San Pedro trying to make a living are restricted by the type of gear necessary, the weather factor, and boat size and it is much easier to be rigged out for flats or reef fishing and there is more of a demand for.
If it is your sisters dream to catch the big one, then tell her I said go for it and, to get her motivated, I will paste a link to my sister's dream that she lived a few years ago on one of the many tournaments we have fished in Belize. http://ambergriscaye.com/pages/town/fishdeep.html
Also, may be a good chance that our all female fishing team will be back in action in February for the International Billfish Tournament since we are unable to fish next week.