Is Her Schizophrenic Drug Using Brother in Law Dangerous to Her Kids?

I can't answer whether this gentleman is dangerous any more accurately than this: Every human being is dangerous in some circumstances. Mental health issues, including schizophrenia, do not typically make people with them more dangerous with people without them. However, some people who use drugs become more dangerous, either because of the effect of the drugs or because of the effect of the cultures they sometimes engage with to obtain drugs.

I am not a psychiatrist, and I do not know the man in question, so I can't state for certain how his medications and marijuana use might interact. I have seen people for whom that was a very bad idea, and others for whom it posed little problem. In general, mixing drugs without discussing it with your physician is a bad idea. 

People with schizophrenia can be dangerous when off their medications, if their delusions and hallucinations lead them to paranoia and fear. It is pretty common for police officers to mishandle mental health situations, so I cannot speak as to whether this man's arrest for assaulting a police officer is a good example of his 'violent tendencies'.

That said, the best predictor of future actions is past actions. If this man has been violent to family members in the past,he may very well be dangerous to them in the future. Your sister may have thought all of this through and has a good understanding of setting boundaries and dealing with mental health crises with him, or she may not.

If your sister feels unsafe, then she might want to have a discussion with her husband regarding alternative arrangements for her brother in law. She might also want to have a plan in place in case of a mental health emergency. 

I strongly suggest that you and she contact NAMI (National Alliance on Mental Health) on their national help line at  1 (800) 950-NAMI (6264) to find a local chapter for assistance and perspective on this situation. If you can find an organization teaching Mental Health First Aid in your area, that is also a good resource. 

Your sister has taken on some major responsibilities, between her children and her disabled brother in law. If you can offer her support and respite from her caretaking tasks, that will probably be a tremendous help to her. 

Good luck and I hope things go well for everyone involved.