If you find teh 24-70 is a little too short, or that at f/2.8 it is too slow, then an 85mm might be the ticket. I've owned the 85mm f/1.8D and 1.4D Nikkors, and the Sigma 1.4. I kept and still use the Sigma. It's my favorite portrait lens. I have not used either of the G Nikkors.
In terms of price, the Sigma is less than either 1.4D Nikkors, and about twice as much as the 1.8 Nikkors.
The next question, of course, is whether what you are shooting will work the narrow depth of field of either f/1.4 or f/1.8?
You can overcome a slow lens with higher ISO, but aperture is forever.
When I bought my D800 I knew that it was time for some new glass. All but one of my full frame lenses was bought when I was still shooting film on my F100.
I have the 85mm f/1.8 G. I was considering the 85mm f/1.4G but after doing a number of test shots back to back I decided that the f/1.8G would meet my needs, and leave me part way in my savings for either a wide prime (24mm 1.4) or a sharper wide tele. The owner of my local Nikon deal had both and was happy to let me test them.
I find the 85mm f/1.8 is sharp, and the contrast is very nice. With the D800 the narrow depth of field is quite apparent.
I had a 85 f1.8 AF (non-D) and used it with my film (F5, N80) and later with DSLR D70. Indoors was a problem for me because it was too long especially on my D70 (due to crop factor). I later sold it due to non-use.
After I upgraded to D800, I re-purchased this FL but the newer 85 f1.8G AFS, because I like the creamy bokeh of this lens and perfect for portraits. I like it a lot and much improved over the non-D. It is still sometimes too long in my small apartment room. But I mainly use it outdoors on a lawn or out with the kids in a park.
You need to decide how small/big is the "indoors" in your case.
Let me say upfront that while I have heard good things about the Nikon 85 f/1.4G, I do not have any first-hand experience with the lens. I am not an 85mm focal length kind of guy--for me this is the second (i.e. much less used) lens in a two or three lens travel kit. For that kind of usage, the extra 1/2 stop speed, weight and price of the f/1.4G vs. the f/1.8G were not attractive to me, and I've been very happy with the shots I get with the 1.8G. (The 35mm focal length is where I chose to step up.)
Since you specifically cite indoor/low light usage, the extra 1/2 stop may be the difference between getting the shot or not. If you need the f/1.4 aperture, then you need the f/1.4 aperture, and that's all there is to it. If cost is a factor, then I think a second-hand Nikon f/1.4D is not a lot more than the f/1.8G, and the f/1.4D is still highly regarded for its rendering of out-of-focus elements (I refuse to use the "b" word ).
This came off Tom Hogan's site from a post he made yesterday. The first number is regular price, the second is the current discount and the third is what B&H is advertising the lens for.
85mm f/1.4G US$1699 US$200 US$1399 Look at the price difference between this and the next lens. Is it worth it for 2/3 of a stop? No.
85mm f/1.8G US$499 US$100 US$396 A huge bargain at the new price. Especially compared to the f/1.4G. Most of the optically for very little of the price. This really should be the 85mm prime choice for most shooters.