How come Devdutta is also Buddha's brother-in-law? In this wiki article it says Devdutta is his Brother in law but how?
Marrying within the family was common in many parts of the ancient world, especially among royalty. The Buddha’s family was no exception. 

Yasodharā (who married Siddhārtha who went on to become the Buddha) and Devadatta were children of Suppabuddha. Suppabuddha was Siddhārtha’s uncle, being the brother of the sisters Māya (Siddhartha’s birth mother) and Pajāpatī ( his stepmother who actually raised him after Māya died at childbirth ). 

So Devadatta was the Buddha’s cousin, brother-in-law, playmate/rival while growing up, follower and ordained student after Siddhartha was enlightened, an important leader in the Sangha*, a causer of a schism in the Sangha, a would-be plotter of his death.  A complex figure  who has been caricatured and narrowed down in the eagerness to paint everybody as either hero or villain.  A book that discusses this is Buddhist Saints of India by Reginald Ray 1994. 

As an aside, it was said that Devadatta’s father, Suppabuddha also was pretty antagonistic towards the Buddha, first because of anger that he had abandoned his beloved daughter to go forth as a samaṇa and later also on account of tensions that his son had with the Buddha.