Depending on your health benefits, you may be able to visit with a psychologist with whom you could discuss this problem, and have the cost covered. They could provide you with some additional information regarding your IL's behavior and signs to watch for that might mean danger. They cannot give legal advice, but could point you in the right direction.
You may also want to contact a social services group such as the CAP agency. Again, they may not be the final person to talk to, but they have a lot of experience with domestic conflict and can point you in the right direction.
I have a similar situation with my own mother. The very best advice I received is WRITE EVERYTHING DOWN. Start a journal or notebook where you date and detail each incident, the times they make inappropriate/frightening comments, and who was there (e.g., if she said these things to your children, or in front of witnesses). Be as precise as you can - if you remember the comment verbatim, quote it. If you heard from a second person (e.g., your husband told you something that was said to him) indicate that. And very importantly, write down anything you or others say to them as well.
This information will be very useful if there is ever any legal action on either of your parts, and/or if you do speak with a counselor, etc.
It was also very strongly recommended to me that I keep this notebook secret from my mother (you ILs, in this case) - that probably goes without saying, but it's important.