If you are in immediate danger, call the police. Don’t be afraid to and don’t be put off by family members trying to guilt you into not doing so. It sounds like you are in a toxic situation where there are minors in an unsuitable environment, unfortunately if you do anything about it you will very likely be blamed or shamed by the family if they support the person that you are fearful of. Don’t take it personally, it is just what happens when people try to protect somebody, even if that somebody is an abuser. Call the police as many times as you need to in order to establish a pattern if the abuse can be explained away by the family.
Record in any way you can what is going on. Cameras and voice recorders don’t lie and if you need to get this person committed then this will stand as very good evidence of why. When it comes down to two opinions, authorities will go with the most convincing and if your family sounds more convincing than you do then you’ll be on the back foot. I was in the opposite situation where my wife was diagnosed with sz and feared me abusing her so called the police many times. Having a factual account of what happened each time stood as a backup if I was ever arrested, fortunately it never came to that.
For your own emotional wellbeing, you might like to look around in your area for a martial arts training course- if you are able bodied that is. This may give you more confidence to handle situations, or at least it will take your mind off things.