a couple of serious things can happen. Like Commonsense was saying in smaller churches sometimes you can not help it, but can cause alot of problems. I have seen it personally. My previous church had been in existence for 30+ yrs and one of the pastors family members was the treasurer. Now it is someone else, because thay family member left the church. The new someone is not family.
Over the years many people have questioned the pastor and family about some of the money. The tithe report is never correct. The fact is that the tithe in cash is never, ever listed on the reports. One yr it said I paid 1 thousand the whole yr in tithe. I am not lying when I say I paid way, way over that. So if a family member does the money stuff it leaves no room for error. If it is handled by someone else then there is a safety net incase of questions. The fact is that no sufficent evidence has been showed to prove this former pastor innocent, but not enough proof to take out a case either. It is all close to the vest.
More truoble than it is worth. For the smaller church it might be worth hiring a small firm of cpa to manage the funds. But some of the ministers like doing it themselves so they can skim off the top. But better safe than sorry.