God bless Lauren;
From what I have learned in God's Word and ministering over the last 34 years, some mental illness is caused by devil spirits, but not in every case. It can also be due to an injury or chemical imbalance or other cause. But we can see from the Gospels, for instance, Jesus ministering to people and then we read afterwards that they were "in their right mind." So sometimes the cause is spiritual.
I know that some believe that a Christian believer cannot be plagued by demons or devil spirits, but when a person gets born again, that new birth does not affect the mind. God does not take our free will. That is why God says we are to renew our mind [Romans 12:2].
If God controlled our minds, I am pretty sure we would all stop sinning.
What makes a person suseptible? Well, the true answer is, we can all be suseptible. And certainly, tragic events can lead to a spirit of depression and other spirits that can affect the mind.
But, I want to stress, that as born again believer's, we have Christ in us [Colossians 1:27], and we can be more than conquerors in every situation [Romans 8:37]. We are never alone. God's power is one believing thought away [Ephesians 1:19].
God did not give us a spirit of fear, but of power, and of love, and of a sound mind. [2 Timothy 1:7]
Your greatest weapons are the sword of the Spirit [the Word of God; Ephesians 6] and prayer.
Continue to pray for your loved ones and lovingly share God's Word with them. The Word of God is living and powerful. If God be for us, who can be against us?
I pray with all my heart that this helps.
You and your husband are God's best.