Uh-oh...I THINK you got it backwards. The sink shouldn't drain into the head intake line...that will result in using gray water to flush the toilet, which is bad idea because gray water is full of soap scum, dirt, body oils, toothpaste, shaving cream maybe...all things that can really "gum up" a toilet pump and can actually be damaging to the rubber parts in it..
The sink should drain normally out its thru-hull. The toilet intake line should just be teed into the sink drain line below the waterline as close to the seacock as possible. That allows you to flush normally with sea water (except when you find yourself in waters where you don't want to), but also provides a safe source of CLEAN fresh water to rinse all the sea water out of the whole system--intake line, pump, channel in the rim of the bowl AND the toilet discharge line before the boat sits...just close the seacock, fill the sink with CLEAN water...flush the toilet. And it provides an easy way to winterize the toilet tank too.
I included detailed instructions in both my books (see link in my signature below).
--Peggie
"If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein