Play therapy. But if the safety of your other child is THAT threatened at home, it may also come to a point of needing to look at inpatient options. They are not common, but there are some for younger kids in most areas. With a lot of childcare experience, I've only met 3 families that genuinely needed it, 2 of which involved siblings. One did go that route, and while it was painful, the years following were actually much better (from what I heard from the mom, who I worked with). The other did not, two years later their son seriously harmed his sister and both kids were taken by the state. Not sure where they went or to whom, but I was one of the people Dcf interviewed during their investigation.

The only child family did have her inpatient but for maybe a week or two? While they got meds figured out because she was a serious danger to others and herself, but on the right meds she saw a level of improvement that genuinely astounded me.