Scared to leave

Hi ladies, I wanted to come here for some advice. My sister has been in a 5 year relationship that she’s been scared to leave for some time now. She finally tried to cut ties gently at first with taking a “break”. Now the guy is seriously acting scary after suspecting that she’s been talking to someone else. He’s calling/texting her nonstop and goes from extremes of cussing her out in the worst way to asking her to hang out. He’s even threatened to find the guy and burn his house down. This is my younger sister so I’m so scared for her and even for the rest of us in the house. The guy is a gun owner and has scared her with them before. Idk if the weird bipolar texting/calling is just his way of dealing with the ending relationship or if this is a serious matter for which we should involve the police. Thoughts?