Today I read that Ekalavya (student of Dronacharya who practiced in front of Dronacharya's statue) is the cousin of Lord Krishna and Krishna killed him.
How are they cousins?
Yes, Ekalavya was Krishna's cousin. Specifically, he was the son of Vasudeva's brother Devashrava, but he was raised by the Nishadas, as described in this chapter of the Harivamsa:
The fourth brother of Vasudeva, namely AnAdhR^iShTi begot on his wife Ashmaki an illustrious son by name NivR^ittashatru, and Devashrava, the third brother of Vasudeva, begat a son by name Shatrughna. There is hearsay that Devashrava had another son who for some reason is discarded in his infant stage, and brought up by tribal people and they call him as Ekalavya.
It's not clear, at least to me, why Devashrava abandoned Ekalavya as a baby though. In any case, this makes it all the more tragic that Krishna had to kill Ekalavya as I discuss in this question.