I've accidently poked my cousin in the eye! Red now. Do we tell mom?

Probably. If your cousin's eye is red, it will be seen as such. There are many outcomes of a traumatic hit to the eye from benign hemorhage, mild inflammation to more severe issues inside the eye. Can your cousin see the same? Is there much pain? Is the eye light sensitive? Your mom will probably notice these things if she sees your cousin. Your aunt/uncle might also.