I had not noticed the list of lenses in your signature. You don't need an 85mm prime. That's a waste of money at this point.

Either the 28-70mm f/2.8 or the 70-200mm f/2.8 will serve your needs, depending on focus distance and the framing you want.

At least two of us have hinted fairly heavily that lights would be a better way to fuss with gear acquisition. With two speedlights (and the SB800 is a good one) you have a start.

Extra lights are cheap if you want to go easy on the pocket book. The results are just as good as with better equipment. The advantage is easy of use, not better images.

You need two main lights, one powerful light that can deal with a softbox. Then you need a fill light, that need not be as powerful. Another pair that also need not be much of anything at all are used for a hair light and one to light the background. The SB800 is good for the main light, and the others can be older used manual lights with optical triggers. Vivitar 283 and 285 models are common, and along with Nikon SB24 are under $50. SB28 and SB26 models are under $100. Brand new models from various Chinese distributors are running everywhere from $40 on up.

So extra flash units is not the problem, it's outfitting them with a light stand and whatever kind of light modifiers are reasonable... the hair light needs a really tall stand and some kind of a snoot and grid. The side lights need medium height stands and barn doors. The main lights might be a softbox on one and a large beauty dish on the other, and they need fairly tall light stands. The background light can use a very short stand and needs something to hold color gels.

The total cost is probably about the same or less than a good 85mm f/1.4, but the effect on your photography would be huge. (You will have a very hard time, shooting studio portraits, seeing a difference between shooting with an 85mm f/1.4 and that 70-200mm f/2.8 set to 85mm.)

You don't need an 85mm, in relatively small space like you have, it will be very limited.... mostly a head and shoulders "tight portrait" only lens.

Besides, you already have one of the best possible portrait lenses for use on a DX camera.... Your Sigma 50/1.4. (Even better than the f2.8 zooms.)

Someone mentions a 60/2.8 macro, which would be no better than using one of your f2.8 zooms.

Instead, if money is really burning a hole in your pocket, Tamron SP 60/2.0 is great both for macro and portraits. It's f2.0 aperture is a full stop larger than the f2.8s.

These large apertures are not so much about low light shooting, as they are about controlling depth of field, being able to strongly blur down backgrounds and make the subject really pop in portraits.

Note that like most macro lenses the Tamron 60/2.0 is not particularly fast focusing. It's fine for portraits and macro.... but not very usable for sports/action.

Here is the URL for a group of calculators. DOF is the most often used, but scroll down to the one labeled "Dimensional Field of View Calculator"

http://www.tawbaware.com/maxlyons/calc.htm

Your D7100 has a crop factor of 1.5x, so at a focus distance of 12' it will provide an image of an area that is about 3' 5" by 2' 3". You might squeeze two people, side by side and close, into a 3' 5" wide place. And the 2' 3" height is plenty of room for a head shot, and with care you might get a head and shoulders.

And that is the most you can get! The problem is that 85mm is about the right focal length in that space for a full frame sensor. You need to either also buy a D610 or get a 50 or 60 mm lens. Another thought is either a 24-70mm zoom or the 24-120mm zoom (my choice for a full frame sensor).

But of course the really big issue is portrait lighting. You need at least two lights on the front, and at least two lights on the sides, plus a hair light and a back light. Six different strobes! (Four if you have to skimp, and skip the side lights.)

GAS about studio portraits is a boundless endeavor...

D, with a crop cam at 14' you can of course get head and shoulder shots but not a lot more. Maybe even waist up but that's about it. Do you have a doorway that you can use to shoot through? The lens will be very usable as long as you don't think you're gonna get full body shots. You can of course get tight, very creative shallow DoF shots too.
Is your room 14' from wall to wall? Because you also want to be pulled away from the backdrop much of the time which will kill even more space.
I currently take 1000's of shots with an 85 but its on a FF and I've got room to move back. With limited space you may be forced to use a zoom. I used a 24-105 just last week for portraits, I hated it, but it got the job done!!! Good luck!
SS