It isn't We Need to Talk About Kevin. I'm hesitant to declare an author has already written her best book, and years ago at that, so I won't. I'm sure Shriver may also be tired of hearing about said prior work, but maybe not. I know I'd be damned pleased if I'd written it. But.
Big Brother was certainly readable (I started it one afternoon, and then stayed up 'til 3 in the morning after company had left finishing it), and enjoyable. I'll say, Shriver has a penchant for unwieldy last names in characters. Also in the infinite quirks of family units, both in makeup and in interpersonal relations.
Shriver also seems to have a good deal to say about dreams, and foolishness, and what works for an individual in his or her life. I think that might be some of the overarching themes for most of her works, really; individuality. We are all stumbling together, shoulder to shoulder, yet alone, through life. Sometimes we reach out better than others. Sometimes we push away. These are the choices we make for ourselves. We cannot make these choices for others.
Or, to be less philosophical, Big Brother is a "what if" book (As so many are). What if somebody's big brother came to visit, and _____? What if he didn't ____?
I do recommend it, especially if you've read other books by Lionel Shriver. I daresay a certain taste might be required for her books to resonate just so, but she is a good writer, with a solid vocabulary that delights me regularly.