Nate - there are several grades/weights of Sunbrella Fabric. The weight used for awnings starts at a weight of 9.5 ounces per square yard. It is a solution dyed acrylic that "breathes", is highly water resistant, an UV resistant. I have used it in many projects and would recommend it for an interior window covering if is is NOT pleated, i.e. as a flat covering. I have sewn this fabric with an older Elna machine, using the longest stitch length.
Sunbrella makes a furniture weight that is 8.0 ounces per square yard. It is also solution dyed acrylic with UV resistant properties. This weight could be used for interior curtains with minimal pleating, depending on the pattern/weave. I have found that several of the furniture weaves in this collection have a good "hand" that facilitates pleating.
Sunbrella does make a "sunbrella shear" fabric that is specifically designed for drapes.
Buy 1/2 yard as a sample for color as well as a practice sample.
I did a post a while back (will look for it) that has some links for online Sunbrella vendors. In my experience, the cost of Sunbrella is not significantly greater than other types of fabric. Remember, if you do embark on a curtain project, the cost of labor outweighs the cost of material. If you are making curtains and investing your aunt's time, get a fabric that will wear well.
If your aunt is making the curtains, I'd really get her
input on what type of fabric that she's willing to sew. Since she's the sewer, she should be able to steer you in the right direction. She's going to have to sit for hours with the fabric, cutting & sewing. When my sister was planning her wedding years ago, she chose a dress pattern for the brides' maids & matron of honor (me) as well as bought the fabric for these dresses without first consulting the seamstress (me). The fabric didn't suit the pattern (was a pretty color though) . . . the dresses happened (as did the wedding) . . .
Show your auntie some pictures of what you have in mind - ask if she has a sample fabric weight for a suggestion.