Are there still descendants of Aaron serving as priests in Jerusalem?
As you are asking on a Judaism site, I assume you want the Jewish belief on the subject...
There are many Jews today who are descended paternally from Aharon the High Priest. How they have that knowledge and whether they can prove it genealogically is another question.
Do they still serve in Jerusalem? Well, the Holy Temple is no more, and it will not be rebuilt until the coming of Moshiach, so they don't serve there.
You may wish to read this Wikipedia article's section dealing with the modern day applications of priesthood.