>>363
What are your motives? Fuck and forget? Relationship?
If you're serious, if you're on good terms with your cousin, you could ask her (or him?) about it, and you could arrange some sort of freetime activity – hiking, amusement park, concert, movie, stroll in the park, a chat somewhere, whatever – that all three of you could enjoy.
Just going at her out of the blue could come off as being desperate. If you are sexually deprived, you should rub one out and clear your thoughts first in my opinion.
And please don't do it on the single assumption that she's easy. It devalues her, and by hoping that, it devalues you as well. Worth and respect are both earned and made.