Nope they are not related by blood. Her mother just said Judas was her uncle for idk what reason. Maybe to trick her- or a forewarning to Tara to not have any romantic thought toward Judas. Since, her mother, the Queen love beauty but she hated beauty that she couldn't get. Similar to how she hated Tiana, the Fairy Queen, who have red hair and otherworldly beauty, she might be afraid that Tara can get Judas, who is a beautiful mage, whom she couldn't get.
I don't know who Tara's father is, but Judas said in the beginning that Tara looked nothing like Adelaide but looks so much like her father. Tara has magical power which she inherit from her father's gene. Her power awakened at the age of 14.