Mum confused on how to discipline her 21-year-old daughter

This can be a confusing one.

When it comes to your older children, there's not much you can do when it comes to them acting out. That's the problem with this mother. Her older daughter hit her younger daughter and she doesn't know what to do. She wrote on Mumsnet:

"DD2(13) is really really cheeky and rude especially with DD1(21). apparently DD2 was very annoying and broke DD1s makeup thing on purpose. DD1 then hit her very hard on the head and told her to "f*ck off". "I stopped her as she she was really angry, and DD2 is crying. I do not condone any physical punishment so i was mad and yelled at her...and DD1 said "if shes a brat then she will get a whack" and she said if she is annoying she will hit her again... I don't know what to do. DD1 is 21 and an adult so i did nothing and i did not punish her. AIBU?"

Many mums replied to the post with advice. One said:

"I don't think you can punish her as such; she's an adult. But I wouldn't condone violence in my house. She's an adult, if she doesn't like her sister's behavior she can live on her own. What are you doing about the younger girl's behavior?"

While another wrote:

"You can’t punish her but you can say no physical aggression under your roof (so she moves out if she wants to belt people). "Why was dd2 touching her stuff? Do they have separate rooms? If so Dd2 needs to stay out of her sisters room. If dd1 is leaving her stuff lying around elsewhere she needs to accept it will get touched/May get broken and keep it in her room."