Welcome to HealingWell. I am so sorry about your Father-in-law. Yes meds can interact with other meds but if both meds are in your system I am not sure if the time they are given factors into the problem, good question for his physician?
His antidepressant may not be the right one for him. Each person is sensitive to the AD meds in thier own way. What works for one may not be the best choice for another patient.
I hope your family is able to find the cause of his behavior.