This is obviously not your sister's fault, but rather her boyfriend's
fault. I think you have every right to be exercised about this, as it
bodes poorly for future family relations should this relationship turn
serious. (Remember, the key to the universe is always having someone to
blame, so you're halfway there.) Go to him, man to man, and tell him to
insist to your sister that you be allowed to watch the show.
Or watch it and lie about it later.
I won't tell.
jms