There are two kinds of families in The Westing Game: the family you choose and the family you're born into. Westing doesn't just leave his estate to a relative; he creates a game of strategy that will help him find the best heir possible. If his estate ends up with a relative, that's great, but it's not a requirement. Similarly, Turtle forges a strong relationship with Flora when she realizes she won't get the kind of maternal care she needs from her own mother. In contrast, though, the sibling relationships we see in the book are really tight. Theo takes great care of Chris, and Turtle looks out for Angela. What we see there is a lot of love and support.
While family bonds can be a strong support for some characters in The Westing Game, it's through the unlikely friendships they form with their teammates that they create even more supportive kinds of family.
Family inspires the strongest kind of loyalty in The Westing Game – even when family members are embarrassing or act stupid, their relatives stand up for or take care of them, even if it means harming themselves in the process.