This has always been a good debate, it ultimately depends on what kind of learner your sister is. Think of it like the Stick Shift vs Automatic comparison. The stick is harder to learn, but once you learn it, you can handle just about anything vs the automatic which is simple, but you still have to eventually learn a stick if you want to drive those types of cars. Same with Taildragger vs Trike. If she is a real patient learner with a a hankering for real challenges, stick her in a tail dragger. If she were planning on just learning in FS and not pursuing a real certificate, I would go ahead with the tail dragger, she will be able to enjoy a wider variety of planes if she does. However going for a real certificate, I am just guessing but probably at her age she isn't planning on a major flying career or serious bush flying so I would probably go the trike route via a 172 or somthing.
Tail-Dragger vs Trike aside, I would just use FS, find somthing with a decent flight model, then you do the takeoffs and landings first, let her fly around, get used to what the controls do, then feed in TOs and landings.
Agree with Sundog that RealAir's upcomming 172 should be an excellent training tool. Baytower's RV-7 would also be a good bet since it has both Trike and TD versions, she can decide which she likes better...lol and it has a lady pilot, she might like that.

Being a Navy guy you might enjoy seeing her flying around in Ricardo's beautiful Navy RV scheme:salute:
I always thought the RealAir Citabria would be a good teacher as well, it has a great foward view of the horizon with a view of the main flight instruments (which can be taken out) for learning pitch attitude relationships to changing flight dyanmics. The A2A J-3 would be excellent for learning the absolute basics...it really made me a better tail dragger pilot in FS. You probably would want somthing with the basic 6-pack of instruments tho so she can really familiarize herself with their function.
If and once she commits to going for a real certificate tho, I would keep her away from FS other than for memorizing checklists and proceedures because it can teach bad habits that will be hard for her and her instructor to break....this would cost more $$$ for her. If she goes on for an IFR ticket then FS is great for mastering instrument scans and navigation/approaches.
Cheers
TJ