Everyone's family dynamic is different.

No way, no how would I farm with my father.  In fact, when he inherited his farm ground the ink was still fresh and he sold it to his brothers.  I farm with my uncles and we all get along just fine.  That's just our family.

Personally, the father in law will likely not be the problem.  If I had to venture to guess the son and the son in law will be the issue at some point.  But again, without knowing much of who they are and the dynamic of their relationship it could be hard.

Either way, I would not give up an established, successful business to farm unless there is a clear understanding there will be equity built.  What I see happen here is the son stays home to farm, runs the farm with dad for a small living wage for 40 years, the dad passes away and then everything gets split x ways between the siblings.  I've seen it at least a dozen or better times.  The son has  to buy out the siblings and his sweat equity has  0 value. Then, at that point there's hard feelings all the way around. 

That being said, farm partnerships here almost NEVER work out.  Remember, that's "Here". I can name a few that do but I can name a lot more that dissolve.  The ONLY way it works is if the land and equipment stays neutral in the LLC so no one owns it and then the partners draw an equal salary.  Otherwise you will inevitably run into the situation of who does the marketing, who does the equipment, etc.  As stated, when the FIL is still around, he will likely be making those decisions.  If he passes will the son take over those responsibilities?  Does he care enough to help is BIL along?  Money does really weird things to people.

FWIW, I could easily farm with my BIL and FIL  No problems.  But I would go into expecting nothing other than a wage for helping out.