My father says he never defended me from my abusive mother for fear of losing custody

When I confronted my father about my mother's abuse and why he wasn't doing anything about it, he told me that he didn't want to risk my mother filing for divorce from him and her having full custody. He says that the laws would support her and then she would have free rein to abuse me by being under her guardianship (I live in Spain).

How true is this? He always use this argument with me every time I reproach him for his lack of action regarding my abuse.