I am worried for myself over this VOS so how did relative know?
What will do, about living with your abuser? Please don't blame yourself for this, or for seeing someone else, living with someone who is so extremely controlling has long term damaging effects on your sanity and alters your thinking of what's OK and what's not, and generally instills an over-inflated sense of loyalty/sympathy to him. You clearly didn't want to see OM hurt and wanted to try to explain to him, and realised the huge difference between the ways you were being treated by dh/om.
Yes, easy to get private detectives, easy to spy, loads of spying software, and you now know this.
If you phone paladin, or the Suzy lamplugh organisation, or the national stalking helpline, they can tell you the implications (dangerous/violent) of his controlling/stalking behaviour. This was not because you cheated. How comfortable would you feel installing stuff on his phone, listening to his calls, watching him going about it private life unaware he was being spied on?
It's all confidential, so no-one will know, bit you will learn the facts that you need to be armed with about who he really is. No family would want you to be trapped in this relationship, not family that's worth anything anyway!
Inform yourself. Call WA
I hope you are OK, do keep posting so you can work through this for yourself