Impact of low quality analog RCA interconnects

There's really only one way to find out... sibilance could come from your source before DAC. For interconnects, I recommend either Blue Jeans LC-1, canare L-4E6S, or Signal Cable Analog One or TWO.
With these, you are sure to get really good cables at the right price. Of course, you could spend hundreds more for better (hum) or for worse, i.e. AQ, Kimber, ZU, or iBELIEVE xr26gold with super secret B3 technics.

Smoe cables can work to reduce sibilance. All you need to do is search for warm sounding cable using the search feature of the forums.
That said the problem of sibilance is more likely a problem with your electronics and tweeter type. In paricular some SS amps have a harsh digitis that is particularly pronounced with some metal tweeters. There has been a lot written about that also.
Back to cables if you are going to upgrade from the free radio shack cables to anything well known for warm soud like certain Cardas cables be prepared for sticker shock. In other words most well regarded interconnects are pretty expensive. Even a modest pair of fairly neutral Audience brand Conductors, their least expensive model, has a list price of about $180 for a 1.5 Meter pair. I paid as much as $500 for a 2.0M pair of used Cardas Golden References a couple of years ago. That cable would have a chance at calming down your sibilance it is a warm type cable.

agree with Mechans, my experience is low quality interconnects cause closed in soundstage (car radio sound) and loss of detail.