If this site generates a sizeable chunk of your income then tell her. Put it to her simply and bluntly, in a way that she'll understand:
if I sell this site we'll have some extra cash, but in 3 months time I won't be able to afford to make your next car payment.
Most of the time it's not worth selling adult sites unless there's some compelling reason (besides $) to get rid of them. You'd be lucky to get 3 months of income as a buy price and for a growing site you'd probably be selling it for less than it's worth. It's better to keep them.
