Wa `Alaykum As-Salamu Wa-ramatullah Wa-barakatuh

You only owe them advice in that which is proper. Sometimes you need to make it direct sometimes indirect.

 

Backbiting is indeed a major sin and it makes problems among Muslims which are supposed to be brothers and sisters in Islam.

 

It might be wise to see if your dad can have some input in this too.

 

Almighty Allah knows best.