Can I Fire An Employee Because Of His Brother Repeatedly Harassing My Business? (montana)

Location: Montana, USA

I own a small business (6 employees) and one of my employee’s brother has been causing ongoing disturbances. The employee in question has been with me for more than a year, only over the last couple of months has his brother’s behavior drastically escalated from what before were only calls to my employee and rare visits, which didn’t affect the workplace. But now he shows up nearly every day and often acts erratic and aggressive toward me, accusing me of withholding wages and abusing employees, but most days he sits directly outside the business (literally on the ground outside the door), seemingly to scare off customers. On days he isn’t directly present he makes constant phone calls to the workplace through different numbers. Recently, he also sent threatening messages to me and a couple of other employees.

I’ve reported this to the police, but nothing has come of it so far. They told me they can’t stop him from being near the store as long as he’s not directly preventing people from entering and the messages can’t be directly interpreted as threatening. The employee himself doesn’t seem to think this is a serious issue, despite the discussions I’ve had with him. He says he can’t control what his brother does, but at the same time he hasn’t taken any steps to help mitigate the situation or keep it from escalating. He seems to accept the harassment and leaves normally with that man after work.

This situation is clearly affecting business, chasing away customers and making my staff uncomfortable.

Would I be able to terminate this employee based on the ongoing disruption caused by his brother, or would that likely be considered wrongful discharge since the behavior is coming from a third party?