My dad is being prosecuted for my sisters truancy when she is living with our mother?

My dad is a hard worker, he works over 40 hours a week and trys his hardest to look after us, my sister lives with my mother, mum is much more liberal than dad. I go to school and so does my sister, she is in her last 2 years at school so it is quite important, she has been ill recently but the school cled it as unauthorised. Because of this the school have threatened to give them BOTH a warning, they have both had a warning anyway from a previous truancy when my sister was emotionally distressed so that which would lead them to be sent to court for impending trial. My question is why have they threatened my dad when my sister is living with my mum?