Ninda is bad-mouthing and slandering other people. Chugli is a particular type of Ninda which is done to spoil someone's work.
Ninda should be avoided, even if the Ninda is true but at the same time if need be, then speaking the truth about someone is not a negative Karma but disclosing other people's shortcomings and mistakes, just to hurt the person or just for entertainment constitutes bad Karma. Here are some thoughts about Ninda:
1. Lying about someone is a Ninda e.g. to falsely accuse a person of drinking when that person does not drink liquor is a Ninda and to falsely praise someone is called Ustat. Such Ninda and Ustat must be avoided.
2. If someone has a shortcoming or commits a mistake and we find out about it, we should not take role of a tabloid and tell the whole world about it. This is also a Ninda that should be avoided but if there is benefit to the society in telling of someone's mistake, then there is nothing wrong in doing so. As an example if someone is a thief and is known to befriend other people and rob them, then there is nothing wrong in warning others about such person.
3. Chugli involves backbiting whereby one slanders other to spoil his work. This should be avoided at all cost.
In your case Akaal7 jeeo, I see nothing wrong in telling your relatives about Radhasoamis or other Sant-dummees. This is not Ninda but Parupkaar. If the government warns its citizens about counterfiet currency it is not doing Ninda. Same way, to tell others about counterfiet sants and holymen is not Ninda. To tell others that Khalsa Panth is the only true Panth and that Guru-dummees like Radhasoamis, Sacha Sauda etc are false, is not doing Ninda and Chugli.
Kulbir Singh