Python 3, 53 bytes
lambda n:sum((n+n%6-3)*n%k<1for k in range(2,4*n))==2
Background
All integers take one of the following forms, with integer k: 6k - 2, 6k - 1, 6k, 6k + 1, 6k + 2, 6k + 3.
Since 6k - 2, 6k, and 6k + 2 are all even, and since 6k + 3 is divisible by 3, with the exception of 2 and 3, all primes must be of the form 6k - 1 or 6k + 1. Since the difference of a twin prime pair is 2, with the exception of (3, 5), all twin prime pairs are of the form (6k - 1, 6k + 1).
Let n be of the form 6k ± 1.
If n = 6k -1, then n + n%6 - 3 = 6k - 1 + (6k - 1)%6 - 3 = 6k - 1 + 5 - 3 = 6k + 1.
If n = 6k + 1, then n + n%6 - 3 = 6k + 1 + (6k + 1)%6 - 3 = 6k + 1 + 1 - 3 = 6k - 1.
Thus, if n is part of a twin prime pair and n ≠ 3, it's twin will be n + n%6 - 3.
How it works (WIP)
Python doesn't have a built-in primality test. While there are short-ish ways to test a single number for primality, doing so for two number would be lengthy. We're going to work with divisors instead.
sum((n+n%6-3)*n%k<1for k in range(2,4*n))
counts how many integers k in the interval [2, 4n) divide (n + n%6 - 3)n evenly, i.e., it counts the number of divisors of (n + n%6 - 3)n in the interval [2, 4n). We claim that this count is 2 if and only if n is part of a twin prime pair.