As a cousin, you have to be a little careful. I do not know how young your cousin is or how close you to his/her parents, but parents generally do not like people telling them there is something "wrong" with their kid. They may very well have concerns of there own, discussed it with their pediatrician and are not choosing to inform members of the family until/unless their is a definitive diagnosis.
***Disclosing bias: My husband and I do not have a supportive family and are not disclosing to them. Some have figured it out, or we suspect have been illegally told by gossips in the school system. (We are not sure) so I am looking at this from an unusual perspective. Even if I thought one relative was supportive and would be of help, I would not tell that person if I thought that information would get to anyone else.
As a cousin, your biggest influence will probably be your own interactions with the child, or if the child's parents know of your diagnosis, they might ask you or your parents for advice if they have suspicions.