<human>: Can you condense the text into three sentences or less?

Ever since my younger sister started puberty, my dad has been hounding her on her appearance. If he's not commenting on the way she dresses (she doesn't dress inappropriately for her age or anything, according to him it's not "girly" enough), it's about how bad her acne is getting, or even the way she sits at home. Just recently there was a fight between them that began after he told her for the thousandth time to "sit right" with her legs closed. It ended with him lunging at her with his sandal in his hand, ready to hit her, but luckily my mom and I intervened. 

Lately he's been getting at her about her weight and last night was the last straw, I think. After coming home from work, my mom and I greeted him at the front door with hugs and kisses. My sister follows short after and the first thing he says to her is, "Have you gained weight? How much do you weigh now, like 180? laughs". She has been locked in her room ever since and refuses to speak with any of us. When my dad overheard me telling my mom my sister is sad, he laughed and said, "She's sad? Haha, why?". I'm so done with his coldness towards her.

Now it's the next day and she still hasn't come out of her room. She hasn't eaten all day and has secluded herself from any of us. I'm so worried about her mental state. 

Not only is the way he speaks to her extremely damaging to her confidence and self worth, but it also sounds like he doesn't love her for just her. My mom understands completely why my sister is upset, but I think she's too afraid to talk to my dad about it. She doesn't like making him upset. I am constantly sticking up for her when he makes negative comments about her but he doesn't get the message.
<bot>: My dad is always making jabs at my sister's appearance and I think it's really impacting her confidence. How do I approach him and let him know that was he's doing is not okay? How can I help my sister?