Question

Can some one come and explain to me who the twelfth Olympian god is, im stuck between Dionysus and Hades. Please explain why they are the twelfth and how/when did they become part of the Olympian council. Thanks. :)

Answer 1

Dioynus is usually the 12th. He was a demigod son of Zeus, but since he invented wine he was made a god. Then Hestia, who had formerly been the 12th olympian, gave up her spot as an olympian to prevent conflict. Hades is one of the major gods but was never honored as an olympian. 
It will depend on where you look but usually either Dioynus or Hestia will be the 12th olympian

Answer 2

Generally, Hades is excluded from the Olympian gods because he stays most of the time in the underworld.

Dionysus should hence be the 12th god you're searching for.He's the last god that was accepted as an Olympian god.

I unfortunately don't know exactly why he was accepted as an Olympian god, but I do know that he is involved in several stories where he interacts and/or helps other Olympian gods. He also managed to rescue people from the underworld and bring them back to Olympus.