We don't just get to know characters as individuals in Daniel Deronda; we also get to know them as parts of families. Some characters, like Daniel, figure out more about who they are in terms of who their family is – in fact, one of Daniel's prime concerns is figuring out his family identity. Other characters, like Gwendolen, make major decisions about what to do with their lives based on how they will affect their families. Another thing to think about is how the concept of "family" is not always traditional in this book – Lydia has four children out of wedlock, Sir Hugo takes Daniel in as his adopted son even before he gets married, and Mirah becomes part of the Meyrick family when they take her in.
Daniel Deronda shows us that it's not who you're related to, but rather who loves you and cares for you, that makes up your family.
In Daniel Deronda, blood ties determine family identity.