Do I get a Sigma 18-35mm f1.8 or a Sigma/Tamron 17-50mm f2.8 OS/VC for general shooting?

Do I get a Sigma 18-35mm f1.8 or a Sigma/Tamron 17-50mm f2.8 OS/VC for general(mostly travel, landscapes, studio work and some video work) shooting. I will be using the lens at night and dark situations. I ll probably even use it for event/wedding photography/videography. I have a Nikon D7200 as well as a 50mm f1.8G and a 18-140mm f3.5-5.6 VR lens.

Both of those will overlap your 18-140 to some extent, so you need to determine why you want to get one or the other.

However, if you're committed, I'd bet heavily that the Sigma is the better lens in every aspect. Their recent lenses are very highly regarded. On the D7200 it will give you the 35mm equivalent of about 28-50 (give or take). If that's enough for you, you will almost certainly not be disappointed in the quality.

The Tamron will give you more reach, but as you mention night and dark situations, you want something fast.

My first thought was, why are you using a Nikon for video? You would be better off selling your Nikon gear and going with a Canon 80D instead. For still, the D7200 is a great camera, but for video, it's pretty bad.

If you're going to be using the lens for video a lot, then you have to go with the one that has image stabilization.

Since image stabilization will allow 3-5 stops slower shutter speeds, that more than makes up the difference in aperture size between the f/1.8 Sigma Art and the f/2.8 Tamron/Sigma options.

So while the Sigma Art lens is the technically optically superior lens, I'd still go with one with stabilization instead.

For general shooting, use your 18-140mm. Use your 50mm for portraits. That 17-50mm won't be useful anymore since it's more or less covered by your long zoom. The 18-35mm however, even if already part of the 18-140mm can be useful since it has a low f/number. You'll want that lens on your camera when the sun goes down.