Need advice on a new prime lens for a crop sensor camera

Almost all gear is good to excellent now, even your lowly kit lens.  That lens is quite capable of making "art" photos, the question is usually the quality of the equipment behind the camera - You. All kinds of artsy fartsy (I like artsy fartsy) images have been made with the 50 f1.8.  The 50 f1.8 produces L lens quality images, I won't get into the how but it does.

By the way, Sigma's 35mm f1.4 is an excellent lens.  My advise is hold off, take at least 5000 images with your two lens then see where your interests take you.  Purchasing lenses is fun, no argument from me, I own 9 of them.  But most experienced photo types have bought lenses that just sit in their bag, that lens bought because they thought they could use it, versus they needed a lens that met certain requirements to take the images they were interested in.

The longer I'm in photography (since 1973) the more convinced I've become that equipment isn't as essential as we think they are.

If you are taking inside pics of your sorority sisters, a flash might serve you more than a lens.  But I must warn you, more flashes sit in more bags, virtually unused.  There is a bit of learning curve but your are more than capable to learn how to use a flash effectively.

For natural ambient light inside, I'd first attach the cheapy 50 f1.8 maybe close it down to roughly 2.8 and boost the ISO, you should get some decent pics if the room lighting is reasonable.

Here's how to effectively purchase lenses (or any equipment for that matter) 1) develop an interest in a subject or project 2) figure out what techniques and equipment you need to make those images 3) purchase that equipment that is critical to their image making.

So lets say you decide to take static photos of your sorority sisters sitting in a chair, more a portrait thing to hand out as a remembrance of their stay in their residence.  Well excellent side lighting can come from a large window so you could place the chair near the window and shoot only in the day.  Or you could use flash, a create light from one or more flash units.  But then you have to decide how the flash is to be used, but on a tight budget you could place one flash off the camera on a cheap flash stand, and fire it either remotely or via wire.  You could purchase an umbrella to shoot the flash through, or if on a budget at least bounce the flash off the wall or ceiling or a bit of both.  For these portrait pics, you 50 f1.8 lens will work just fine but stop it down to f4 or f5.6; those f stops are probably that lenses "sweet spot."