The sulfur smell in your bathroom sink is probably caused by a build up of Hydrogen Sulfide in your water supply. Please note, if you are receiving your water from a well, a shock treatment may be required to treat the bacteria in your water supply.
Our bathroom sink had a sulfur smell last month so we called a guy to come out and look at our well. It turned out that our well needed a new filter and a new water softener.
If this occurs only with the bathroom sink, then the problem is related to some personal care item. Some hair treatment products contain chemicals that give off a sulfur odor.