Elsa Einstein, Albert Einstein's second wife, was a relative of his, though much more distant than a first cousin. The two knew each other as children, and she was born with the surname of Einstein.
Albert Einstein's three biological children were the product of his first marriage. Though he and Elsa never had children together, they jointly raised their children from each of their first marriages. By all accounts, Elsa was a major source of support for Albert, and their relationship was healthy and strong.
While Elsa's surname was Einstein, her first marriage changed her last name to Löwenthal. Once she married Albert, her last name changed back to Einstein.