You probably won’t like my answer, just warning you!
Because what you are doing is not working you need to do something different. My suggestion is to treat them like 2 year olds – keep them in your sight at all times. Period. If you need to go to a different room they must go with you. They are not free to play or work anywhere you are not at because they cannot be trusted. Hurting others and lying about it, and a child who has lied for months and gotten away with it, are not going to fix themselves with some quick, easy consequence. You can see that. Which means that the sooner you get drastic the sooner you can hope things begin to change. It also means that you can’t have a few good days or even a good week and then let them go back to freedom and right into their old, lying and hurting each other ways. I would guess that at minimum you’ll need to keep them with you at all times for a few months.
Why keep them with you? Because YOU need to see and hear everything that is happening, what they are doing, what they are saying, etc. Just like a toddler, you need to oversee their behavior and give corrections right away. They will have no opportunity for lying because you are with them and paying attention to what is going on.
Yes. I told you that you probably wouldn’t like my advice! LOL However, I say it in love, this is something that will only get worse if left as things are. A drastic few months now, or even a year of drastic, is better than children who grow into teens that are still lying and suddenly have a whole new world of bigger things to lie about. Yes, it is frustrating to have them lie now. But I shudder when I think of the things a teen who is in the habit of deception can lie about (where they are going, who they are with, what they are doing, the list goes on and on).
((HUGS)) Being the mom is hard! Praying for you!