Not half not full. I'd say 3/4ths.
1/2 for the same mother
1/4 for the father- cause its the same guy, but not.
you are half siblings if one of your parents are different from your sibling. so if your mom had another daughter, but she had a different dad then you, then you'd only be half siblings
They are full siblings. They have the same mother and the same father. Zues was just in his Greek Form when Thalia was born, and he was in his Roman form when Jason was born. He was the same person, just in a different form.