I can't tell for sure, but from the pictures it looks like 3/4 washing machine/garden hose threading on the sink. You could tee off the supply line, have one go to the sink, and the other to a 3/4 threaded to 1/4 compression fitting which you'll connect rodi line to.
Washing Machine/Laundry Sink Y Wye Water Splitter Valve 3/4" Female GHT to 2X 3/4" Male: Amazon.com: Tools & Home Improvement
www.amazon.com
Everbilt - 1/2 FHT OD Compression Brass Adapter Fitting - this product is as easy to use as it is versatile.
www.homedepot.com