My little sister is 9 and she lives with her mum (she’s not my mum) and I live with my dad

Her mum is really wicked and shouts at her and forces her to do, wear and eat certain things. My little sister is such a worrier, one time she came home from school with her uniform covered in muck and my dad told her to take it off and get into pjs so he could wash it and she said “if that was mummy she’d kill me”

I’m really worried for her and the possible verbal and emotional harm (abuse?) her mum does to her.

I’ve brought it up to my dad but he said there’s nothing we can do about it because if we do she would stop us seeing my sister. My dad literally isn’t legally her dad (she’d mum stopped him from putting his name on her birth certificate) even though he is her dad.

Her mum literally hates me and thinks I’m against her and everything. She can’t drive so my dad has to drive her everywhere. She flirts with him and feels his hands and ruffles his hair but he says nothing for fear of her stopping us seeing my sister.

It breaks my heart knowing what she’s going through but she’s too worried to say. I’ve cried about it so much.

I’m not sure if I should tell my mum (she lives in a different country to me) not sure if it’s her business or her problem