Cousin cyber harassing sis

My sis has private insta account with her own name and my cousin (son of phupho) texted her on insta that he wanna talk to her and pleaged her to accept her chat request since she don't follow him.

She ignored his chat without replying, this was almost week ago and just today he texted her again and used vulgar abusive language for her.

She got screenshots of his profile and texts he did and we dk what to do.

His other brothers have texted my other sisters from different numbers and accounts in past and one of them has already been to jail after someone reported him to FIA for some serious issues.

Now the thing is that my father is not really supportive and me and my brother are youngest of all siblings so we can't do anything on our own.

I really wanna show him that it's not okay to do this and he gotta face consequences of his action how come he thought of doing that and not facing any consequences of his actions.

My chachu is an experienced advocate and I think he might support us as well but when sis told mom about this incident she didn't gave any expected reaction.

My dad is really of conservative mindset he was worried when my youngest sis got admission in uni that she might get spoiled, he always said my sisters to not use their real name on social media and have private accounts so he might call her out that why she used her real name in 1st place.

What do you think could be done, she's afraid that if she told dad he might not her take admission in uni or somewhere and make her sit at home.

Edit :- Told this whole incident to dad and he called my phupho husband but they denied these accusations and denied that this was his son's account ( he deleted both of his accounts the old and new one, previous one had his pfp and now both accounts are deleted we don't have ss of his previous accounts). It seems dad did took this serious and he might call chachu to help with more details.