Sinus problems are tricky b/c generally people are aiming to dry their sinuses up - which anythign that will do tht will also have the same effect on yoru milk ducts (milk ducts & sinuses are similar & affected similarly).
However, fenugreek works as a natural secretion thinner - so it will work and has the added benefit of increasing milk supply

Itll drain your sinuses, but wont dry them out.
OTC - tylenol sinus Ive heard. Kellymom.com has a list as well
Lala...